Created attachment 164527 [details]
Error message

STEPS TO REPRODUCE
1. Open Configuration dialog box
2. Select Composer, then Custom Template
3. Add a new custom template
4. Assign a shortcut that already exist in standard shortcuts. In my case, I used "Ctrl+Shift+R". Note, the issue exists even if the shortcut is not enabled. It existence is sufficient to generate the error message.
5. Double-click on any mail, so kmail checks for conflicts in shortcuts.

OBSERVED RESULT
An error message appears. See attachment.

EXPECTED RESULT
Nothing should happen

OBSERVATIONS
The checkbox "Do not show this message again" seems to work as expected.

I assume the issue also exist for Shortcut associated to custom filters.