Is it possible that this only occurs when using Thunderbird?

I investigated this further.

My initial description was wrong.
Duplicates are only shown, if you have different case in the email address.
Example:
christian.mack@uni-konstanz.de versus Christian.Mack@uni-konstanz.de
This does not happen, if you copy an address card to a private address book.
And per standard the local part of an email address is case sensitive.
So this is done correct by SOGo.
Sorry for that :-(

Different case can happen, when you store your used email addresses from Thunderbird in SOGo, or when you type in an address manually into the address card.

Note:
Thunderbird/Lightning always shows only one address. It seems to be case insensitive on invitations.

Ok so is there a problem or not?

I just don't understand from your explanation above :-)

There is a problem for us, as you can not distinguish, which one of the email addresses with different case has free/busy information, and which one doesn't.

I said "for us", as our email addresses do not differ in case.
There are email servers where different case are different mailboxes.
Because of that showing both in the search results is correct.

The solution would be, to mark invitees with free/busy information available in the search result list (eg. other background colour or in bold).
With this the user can choose the one he wants, and knows is the correct mailbox/person.

Fixed.

See https://github.com/inverse-inc/sogo/commit/fcddeb4224308f80184000ad34add7ffc3dcf3c2