Hi partner,
First of all, I tried in a pure environment and there is only one ribbon in UCI sub grid.

So I suggest that you could use Ribbon Workbench to check the buttons in quote line subgrid.
I created a new solution only contains quote line entity and open it in ribbon workbench.
We could see that in subgrid, there are 2 add buttons.

According to the Tool tip description, we could know that the add button in UCI in default is "ADD NEW {0}"


So you could check the another add button display rules or hide it directly and try again.
Best Regards,
Leo