AutoMoc: Do not list moc macros multiple times

When working on a project with a nested dependency tree, a macro
can be added multiple times. For example in a project I am working
on, in the warning "includes the moc file [... ] but does not contain
a [...] macro" the macro list contains 127 entries but only 25 unique
ones.
66 jobs for !6459 with mocmacrosonce in 33 minutes and 43 seconds (queued for 1 second)
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