unwanted symbol instance in the layout

Hello.

I am user of IC6.1.7 with GPDK180_v3.3.

From time to time when I started to run DRC, unexpected schematic symbols are added, which results in VDB error. 

Of course, when such a case occurs, I manually delete unexpected schematic symbol that I didn't add in the lay out to run drc in a normal way. But it feels cumbersome. 

I don't why such symbols, usually nmos or pmos(one or two), are added in the layout although I did nothing.

Do you have any answer?