Add attributes.diagnostic.do_not_recommend.duplicates

Author: ehuss

src/attributes/diagnostics.md

@@ -663,6 +663,8 @@ The `diagnostic::do_not_recommend` attribute does not accept any arguments, thou
 r[attributes.diagnostic.do_not_recommend.allowed-positions]
 The `diagnostic::do_not_recommend` attribute should be placed on a [trait implementation item][trait-impl], though it is not an error to be located in other positions.
 
+r[attributes.diagnostic.do_not_recommend.duplicates]
+Duplicate instances of the `diagnostic::do_not_recommend` attribute are ignored.
 
 [Clippy]: https://github.com/rust-lang/rust-clippy
 [`Drop`]: ../special-types-and-traits.md#drop