How do I credit an actress who is playing a stand-in for another actress who plays twins?

In a tv series we have a main character who discovers she has a twin sister. The actress playing the main character is credited for both roles, but the stand-in used in scenes where both characters appear needs to be credited.

I have now tried to credit her twice, calling her character "Tinka double" (as the main character is called Tinka) but it keeps getting rejected.

Can anyone tell me how to credit the double/stand-in??