Also note that the %date macro is not honored, and %forgemeta just uses the current date.

The reason the timestamp gets duplicated in %{dist} is that rpmbuild actually parses the spec file *twice* (reason: implementation details, not important here) which the %forgemeta macro (defined in /usr/lib/rpm/macros.d/macros.forge) doesn't account for.  Since it prepends the timestamp to the existing %{dist} value, when it's called the second time during rpmbuild, it reuses the previous value with the timestamp already in it, hence duplicating it.

Fixing this in %forgemeta would be quite difficult (if not impossible) without running the risk of breaking existing specs out there.  The original %{dist} value ("el8" in this case) has to be retained as a suffix so we have to read it first.  The thing is, we can't tell whether the timestamp was already prepended or not due to a previous call of this macro (technically, we can, of course, but that would be a dirty solution).  Upstream, this macro was recently completely revamped and it no longer seems to cause this issue, but we don't want to rebase the macro in RHEL-8 as that, as mentioned, could cause regressions.

Luckily, as it turns out, this can be very easily worked around; just move the %forgemeta call to *after* the %{dist} macro usage, i.e. after the Release: tag in the spec file.  The exact mechanics of why this works are unknown to me - perhaps it's because the Release: tag is only applied during the first spec pass.  I've also seen other package specs utilizing %forgemeta do the exact same thing.  In fact, I've tried this with the gnome-shell-extension-argos-3-5 version of the spec file mentioned in Comment 0 and it fixed the problem.

As for the date discrepancy mentioned in Comment 1, that's actually completely unrelated to this.  In fact, it doesn't even work in RHEL-8 at the moment - the %date macro isn't read in %forgemeta at all.  Instead, the tarball's mtime is used.  Support for %date was only added later and the patch isn't in RHEL-8.  While adding it into RHEL-8 wouldn't be too hard, unless it's going to be generally useful, we'd rather not backport random features.

Therefore, I'm closing this bug as WONTFIX.  If you'd like to see the %date feature backported, please reopen this BZ or file a new one.  Thanks!