|Definition - Using the copular is twice in a row.|
Example - What my fear is is that you will leave.
1. Don't use a double "is" if the subject doesn't contain a copula.
My belief is is that you suck. (bad)
My belief is that you suck. (good)
Here the phrase My belief is the subject, so it only needs one copula.
2. Use a double "is" if the subject of the sentence contains a copula.
What my belief is is that you suck.
In the above, the subject is the phrase What my belief is and the predicate is the phrase is that you suck.