Add a log code NuGetAuditSuppress duplicate items

[Nikolche Kolev]

Duplicate item checking is supposed to raise warnings when duplicates are seen.

When NuGetAuditSuppress was added, NU1505 was added as a warning for it.
This should be a new log code.

cc @zivkan

[Nikolche Kolev]

Marking as P1 since we want it done for .NET 9

[dotnet-policy-service]

@zivkan Issue is missing Type label, remember to add a Type label

[dotnet-policy-service]

Issue is missing Type label, remember to add a Type label