I can reproduce this in a Fedora 32 64-bit container:

dnf install mono-devel-6.6.0-5.fc32.i686

This will install some mono 64 bit packages, but not mono-devel

dnf update

This will try to install mono-devel, and that fails.

I have no idea, why mono-devel 64 bit is now required.

I have no idea, why the 64 bit packages are installed when 32 bit packages are installed.

I don't really know why you need 32 bit packages on a 64 bit machine?

I need the 32-bit package to run 3rd party 32-bit software.

There is actually a difference in those files, so at the moment, we cannot install mono-devel.i686 and mono-devel.x86_64 at the same time.
You can only install both packages at the same time, if the files are identical.

[root@l183-f32 tmp]# diff -uNr 32bit/usr/bin/mono-find-requires 64bit/usr/bin/mono-find-requires 
--- 32bit/usr/bin/mono-find-requires	2020-06-20 07:10:16.000000000 +0200
+++ 64bit/usr/bin/mono-find-requires	2020-06-20 07:04:31.000000000 +0200
@@ -26,7 +26,7 @@
 # Can override .config scanning if specified
 : ${IGNORE_CONFIG_SCAN=0}
 
-libdir=$prefix/lib
+libdir=$prefix/lib64
 bindir=$prefix/bin
 
 # Bail out if monodis or libmono is missing
@@ -36,7 +36,7 @@
 fi
 
 # special case for 64bit archs
-if test "xlib" = "xlib64" ; then
+if test "xlib64" = "xlib64" ; then
         libext="()(64bit)"
 else
 	# (note, this works on ppc64 since we only have 32bit mono)

[root@l183-f32 tmp]# diff -uNr 32bit/usr/bin/mono-find-provides 64bit/usr/bin/mono-find-provides
--- 32bit/usr/bin/mono-find-provides    2020-06-20 07:10:16.000000000 +0200
+++ 64bit/usr/bin/mono-find-provides    2020-06-20 07:04:31.000000000 +0200
@@ -23,7 +23,7 @@
 # Set the prefix, unless it is overriden (used when building mono rpms)
 : ${prefix=/usr}

-libdir=$prefix/lib
+libdir=$prefix/lib64
 bindir=$prefix/bin

 # Bail out if monodis or libmono is missing

[root@l183-f32 tmp]# diff -uNr 32bit/usr/share/mono-2.0/mono/eglib/eglib-config.h 64bit/usr/share/mono-2.0/mono/eglib/eglib-config.h
--- 32bit/usr/share/mono-2.0/mono/eglib/eglib-config.h  2020-06-20 07:08:46.000000000 +0200
+++ 64bit/usr/share/mono-2.0/mono/eglib/eglib-config.h  2020-06-20 07:03:26.000000000 +0200
@@ -26,7 +26,7 @@
 typedef size_t gsize;
 typedef ptrdiff_t gssize;

-#define G_GSIZE_FORMAT   "u"
+#define G_GSIZE_FORMAT   "lu"

 #if defined (HOST_WATCHOS)
 #undef G_BREAKPOINT

We could make the files /usr/bin/mono-find-provides and /usr/bin/mono-find-requires the same, and check for existance of directory $prefix/lib64. But I don't know about eglib-config.h, how to keep that file the same for both arches?

The other option would be, to prevent installing mono-core.x86_64 when installing mono-devel.i686. But I don't know how to do that.

A workaround seems to be:

dnf remove mono-core
dnf install mono-core.i686
dnf install mono-devel.i686
# there is no mono-core.x86_64 installed

That seems to work for me on my test system.

This also does seem to work:

dnf remove mono-core
dnf install mono-devel.i686 
# installs mono-core.i686 and mono-core.x86_64

I am open to any suggestions for a proper solution...

Another clue:

# yum update mono-devel.i686 --exclude=mono-devel.x86_64
Fedora 32 - local packages for x86_64                                                               402 kB/s | 3.0 kB     00:00    
Dependencies resolved.

 Problem: mono-devel-6.6.0-7.fc32.i686 has inferior architecture
  - cannot install the best update candidate for package mono-devel-6.6.0-5.fc32.i686
  - package mono-devel-6.6.0-7.fc32.x86_64 is filtered out by exclude filtering
Nothing to do.
Complete!

I think this is bug in DNF's multilib handling, since I see nothing obiously wrong with mono.spec.  To reproduce:

podman run -it fedora:32 bash
[root@18e528d26668 /]# dnf -y --disablerepo=updates install mono-devel.i686
… [success]
[root@18e528d26668 /]# dnf -y update mono-devel.i686
Last metadata expiration check: 0:00:29 ago on Sat Aug 22 14:50:04 2020.
Dependencies resolved.
====================================================================================================================================
 Package                              Architecture               Version                          Repository                   Size
====================================================================================================================================
Upgrading:
 mono-core                            i686                       6.6.0-7.fc32                     updates                      17 M
 mono-core                            x86_64                     6.6.0-7.fc32                     updates                      17 M
 mono-data                            x86_64                     6.6.0-7.fc32                     updates                     4.8 M
 mono-data-sqlite                     x86_64                     6.6.0-7.fc32                     updates                      79 k
 mono-devel                           i686                       6.6.0-7.fc32                     updates                     6.0 M
 mono-extras                          x86_64                     6.6.0-7.fc32                     updates                     491 k
 mono-mvc                             x86_64                     6.6.0-7.fc32                     updates                     483 k
 mono-wcf                             x86_64                     6.6.0-7.fc32                     updates                     969 k
 mono-web                             x86_64                     6.6.0-7.fc32                     updates                     2.6 M
 mono-winforms                        x86_64                     6.6.0-7.fc32                     updates                     1.7 M
 monodoc                              x86_64                     6.6.0-7.fc32                     updates                      19 M
Installing dependencies:
 mono-devel                           x86_64                     6.6.0-7.fc32                     updates                     5.9 M

Transaction Summary
====================================================================================================================================
Install   1 Package
Upgrade  11 Packages

Total download size: 75 M
Downloading Packages:
(1/12): mono-devel-6.6.0-7.fc32.x86_64.rpm                                                          148 kB/s | 5.9 MB     00:40    
(2/12): mono-core-6.6.0-7.fc32.x86_64.rpm                                                           330 kB/s |  17 MB     00:52    
(3/12): mono-data-sqlite-6.6.0-7.fc32.x86_64.rpm                                                    206 kB/s |  79 kB     00:00    
(4/12): mono-data-6.6.0-7.fc32.x86_64.rpm                                                           193 kB/s | 4.8 MB     00:25    
(5/12): mono-extras-6.6.0-7.fc32.x86_64.rpm                                                         204 kB/s | 491 kB     00:02    
(6/12): mono-mvc-6.6.0-7.fc32.x86_64.rpm                                                            182 kB/s | 483 kB     00:02    
(7/12): mono-devel-6.6.0-7.fc32.i686.rpm                                                            274 kB/s | 6.0 MB     00:22    
(8/12): mono-wcf-6.6.0-7.fc32.x86_64.rpm                                                            164 kB/s | 969 kB     00:05    
(9/12): mono-core-6.6.0-7.fc32.i686.rpm                                                             225 kB/s |  17 MB     01:17    
(10/12): mono-web-6.6.0-7.fc32.x86_64.rpm                                                           242 kB/s | 2.6 MB     00:10    
(11/12): mono-winforms-6.6.0-7.fc32.x86_64.rpm                                                      164 kB/s | 1.7 MB     00:10    
(12/12): monodoc-6.6.0-7.fc32.x86_64.rpm                                                            540 kB/s |  19 MB     00:35    
------------------------------------------------------------------------------------------------------------------------------------
Total                                                                                               682 kB/s |  75 MB     01:53     
Running transaction check
Transaction check succeeded.
Running transaction test
The downloaded packages were saved in cache until the next successful transaction.
You can remove cached packages by executing 'dnf clean packages'.
Error: Transaction test error:
  file /usr/bin/mono-find-provides conflicts between attempted installs of mono-devel-6.6.0-7.fc32.i686 and mono-devel-6.6.0-7.fc32.x86_64
  file /usr/bin/mono-find-requires conflicts between attempted installs of mono-devel-6.6.0-7.fc32.i686 and mono-devel-6.6.0-7.fc32.x86_64
  file /usr/share/mono-2.0/mono/eglib/eglib-config.h conflicts between attempted installs of mono-devel-6.6.0-7.fc32.i686 and mono-devel-6.6.0-7.fc32.x86_64

[root@18e528d26668 /]# dnf -y remove mono-devel.i686
… [success]
[root@18e528d26668 /]# dnf -y install mono-devel.i686
… [success—installs all the packages I wanted updated]

In my opinion, dnf update should have the same result as remove/install.  I suggest the mono maintainers reassign to dnf.

The root cause is that the packages contain a file under the same path but with different content/checksums.
RPM errors out in such cases.
This has to be fixed in the packages.

Daniel: Thank you! You were right.
I modified Mono in Rawhide, to have identical files for both architectures.
Now I am able to upgrade from Mono 6.10.0-0 to 6.10.0-1 on Rawhide, and mono-core-6.10.0-1.fc34.x86_64 and mono-core-6.10.0-1.fc34.i686 are installed, but only mono-devel-6.10.0-1.fc34.i686 is installed. I don't get any errors anymore.

I will backport this to F33 and F32.

FEDORA-2020-d7f9fbd8e0 has been submitted as an update to Fedora 33. https://bodhi.fedoraproject.org/updates/FEDORA-2020-d7f9fbd8e0

FEDORA-2020-733020691d has been submitted as an update to Fedora 32. https://bodhi.fedoraproject.org/updates/FEDORA-2020-733020691d

FEDORA-2020-733020691d has been pushed to the Fedora 32 testing repository.
In short time you'll be able to install the update with the following command:
`sudo dnf upgrade --enablerepo=updates-testing --advisory=FEDORA-2020-733020691d`
You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-2020-733020691d

See also https://fedoraproject.org/wiki/QA:Updates_Testing for more information on how to test updates.

FEDORA-2020-d7f9fbd8e0 has been pushed to the Fedora 33 testing repository.
In short time you'll be able to install the update with the following command:
`sudo dnf upgrade --enablerepo=updates-testing --advisory=FEDORA-2020-d7f9fbd8e0`
You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-2020-d7f9fbd8e0

See also https://fedoraproject.org/wiki/QA:Updates_Testing for more information on how to test updates.

FEDORA-2020-d7f9fbd8e0 has been pushed to the Fedora 33 stable repository.
If problem still persists, please make note of it in this bug report.

FEDORA-2020-733020691d has been pushed to the Fedora 32 stable repository.
If problem still persists, please make note of it in this bug report.