I don't think we want to change current behaviour. Altering ExecStart option for service really changes vendor supplied semantics substantially thus in such cases copying the whole thing to /etc is advised. I am tempted to close as NOTABUG.

While there is an error message that gives some indication of what went wrong, I personally think that since the operation makes no sense whatsoever, the overridden attribute should take precedence.

It breaks the promise of systemd that "configuration in /etc overrides configuration in /usr" to not do this. Of course, I could just not .include the system-provided unit and write my own complete unit, but then you lose the maintainability value that including the system-provided unit gives you.
 
An argument could also possibly be made for what I'll call "non-accretive" values in /etc. Whereas the following is perfectly valid in any unit:

[Service]
Environment=FOO=bar
Environment=BAZ=bar

One could quite reasonably expect that if the first line was in a unit in /usr, and the second in an overriding unit in /etc which included the first unit, then $FOO wouldn't be set in the resultant environment - i.e. the content of the second unit really DOES override the content of the first.

I realize that this results in special handling of .include'd units, possibly adding something like '.includeoverride' or something of the sort so that people can opt-in to this change in behavior.

(In reply to Jon Stanley from comment #3)

> unit, then $FOO wouldn't be set in the resultant environment - i.e. the
> content of the second unit really DOES override the content of the first.

I really should clarify this and say that "identical keys in the second included unit override the values in the first, and reset them"

Let me know if this is unclear, I realize my first wording of this was about as clear as mud :)

Closed for reason mention in Comment 2. Changing ExecStart is substantial change how the unit works. ALso this stanza for example uses variables given by Environment= , this could break easily during update.

Btw using .include is deprecated, you should use drop-in files. For the service foo.service create foo.service.d/SOMETHING.conf.