|Definition - Using two successive copulas, such as is or was, when only one is necessary.|
1. Don't use a double copula 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 copula 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.
1. It might be interesting to note that in cases where the subject of the sentence contains a copula, it is possible to offset the slightly confusing subject with a comma. For example,
What my belief is, is that you suck.
I've seen this done in English, but I believe that it's more common in other European languages. Also note that it's an anomaly in that you rarely put a comma between the subject and the predicate.
[Author note: Is it true that the Godfather Parts I and II are double Coppolas?]