kill-do-not-save-duplicates

kill-do-not-save-duplicates is a variable defined in `simple.el'.
Its value is

nil

Documentation:
Do not add a new string to `kill-ring' if it duplicates the last one.
The comparison is done using `equal-including-properties'.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.2 of Emacs.