After duplicate message system picks category expiry date rather than manual defined, the test is as follows:
1. Try to enter a new member
2. Give member Expiry date which is more than defined in concern category
3. If Koha found similar member than pop up a message
4. Select not a duplicate member (New member option)
5. Koha saved the record but the expiry date will pick from category rather than you assigned the member.
6. If the member is new and no message is populated than it will save expiry date as you defined.
Koha version 16.11.07 on debian Jessie installed by package.
Tested following described steps and appears to still exist on current master.