Tracking Issue: Prevent duplicate diagnostic emission of lints

[flip1995]

Description

Follow up to #12374

That PR added the -Zdeduplicate-diagnostics=yes flag to the following test files, to suppress the warning that will now be produced should new lints/tests emit duplicated diagnostics. Old lints should still be cleaned up though. Those are the test files that were affected:

• tests/ui/assign_ops2.rs
• tests/ui/else_if_without_else.rs
• tests/ui/identity_op.rs
• tests/ui/indexing_slicing_index.rs
  • Might be coming from the compiler: Tracking Issue: Prevent duplicate diagnostic emission of lints #12379 (comment)
• tests/ui/manual_retain.rs
• tests/ui/many_single_char_names.rs
• tests/ui/min_rust_version_invalid_attr.rs
• tests/ui/mut_mut.rs
• tests/ui/no_effect_replace.rs
• tests/ui/nonminimal_bool_methods.rs
• tests/ui/nonminimal_bool.rs
• tests/ui/option_option.rs
• tests/ui/ptr_as_ptr.rs
• tests/ui/renamed_builtin_attr.rs
• tests/ui/single_match_else.rs
• tests/ui/single_match.rs
• tests/ui/std_instead_of_core.rs
• tests/ui/suspicious_operation_groupings.rs
• tests/ui/type_complexity.rs
• tests/ui/unknown_attribute.rs

To fix some of those:

1. Pick one test and leave a comment about it
2. Remove the -Zdeduplicate-diagnostics=yes flag from it
3. Figure out the cause of duplicated diagnostic
4. Fix it and write a comment here so that it gets marked as resolved.

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