date: allow multiple -d, last wins

[cakebaker]

When using -d multiple times, our implementation fails with an "argument cannot be used multiple times" error whereas GNU date allows it and uses the last value.

$ cargo run -q date -d "invalid" -d "2000-02-02"
error: the argument '--date <STRING>' cannot be used multiple times

For more information, try '--help'.

$ date -d "invalid" -d "2000-02-02"
Wed Feb  2 00:00:00 CET 2000

This PR fixes the issue.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/misc/stdbuf (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/misc/tee (passes in this run but fails in the 'main' branch)