New Tablesome table is created instead of adding records

Ik keep Tablesome tables for registering who will join the bus to a basketball game.
The input is done via a simple Forminator form.
However in some cases (I cannot see any pattern on when this happens) the record is not added to the intended table, but instead a new table is created in which those new records are added. Sometimes the records are however added in both the normal table and the newly created table!
This makes the use of tablesome unreliable.
Could it be that this happens when we add records with field contents that already exists? For instande a phone number used in 2 records?