There is a weird interaction between SavedInstances and any addon that sends a /played command at login.
SI duplicates any /played command on login.
This interaction bug happens with ElvUI Shadow and Light, Altoholic, etc.

By default SI does not show /played on login.
But any time an addon shows /played on login, SI will duplicate the entry.
Intead of 2 text lines for /played, you get 4 lines because of SI.

In SI options, disabling the track Time /played tracking did not help.
I am not sure what in SI is causing this duplication bug.

Are you sure that it is SI causing it? I'd rather blame Details

Sadly yes, this happens when SI is enabled with any other addon that queries /played at login.
So: SI + Elvui S&L
or: SI + Altoholic.
I get the duplication problem if I enable any of these 2 (ofc I disable all other addons).
The duplicated lines only show if SI is enabled.

If Details! is enabled along with the above:
The first 2 lines for /played are default one.
The 2 duplicated lines will have the additonal "Class" formatting from Details!.
So Details! just changes the format of the duplicated lines, but does not cause the duplication.