Don't show the same help multiple times for object safety violations

[WaffleLapkin]

#116401 made it so that we report all object safety violations at the same time. However, it also results in a redundant note:

https://github.com/rust-lang/rust/pull/116401/files#diff-ea89a7ba6273e575f66679f7c4a10297572773dc805cc27eb6398f7c12527ab8R16

We should not add the same note multiple times.

[matthiaskrgr]

Can we add a debug assertion for duplicate identical help messages? (sorta like we do for "Span must not be empty and have no suggestion" or "Suggestion may not have overlapping parts" ?

[WaffleLapkin]

@matthiaskrgr I guess we could, but I think there are multiple diagnostics that currently do this, so we'll have to fix them first.

[matthiaskrgr]

Did not find tooo much...
inside tests:

for i in `git ls-files | grep \.stderr` ; do ; cat $i | uniq -cd ; done