Address book doesn't accept duplicate mail adresses in a Personal Address Book

[fmh_aigen]

I'm managing some tasks for a club and try to set up an address book for all the members. I found, that TB 128.4.2esr (64-Bit) silently ignores added members, like couples, who use the same mail address.
My only solution is to add another address entry addressing both of them. Not so nice.
Could you please think about issuing a warning if an address is not accepted ?

Thank you!

[Jon]

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[wsmwk]

Please provide more details about precisely the steps that fail.

I have no trouble creating two contacts with the same address, as seen in the screen shot.

[fmh_aigen]

Hi wsmwk!
Thank you for looking into this.

The problem shows up when I try to add both of these contacts to a Personal Address Book. Only one of them shows up in the Personal Address Book. 
When I added a third contact addressing both of them (with the idea of adding just this one to the Personal Address Book) I found, that I have have no control which of the three contacts with the same address gets added to the Personal Address Book.
Matt has shown me a way how I can get around this:
https://support.mozilla.org/en-US/questions/1473592?utm_campaign=questions-reply
and also here:
https://support.mozilla.org/en-US/kb/how-create-and-use-mailing-lists-thunderbird

As TB uses only the email address to differentiate between contacts it would be nice to warn a user trying to add a contact duplicating an existing address.