The thing is, is that...

Has anyone else noticed the "double is" construction in everyday conversations?

For example, one might say "The thing is is that she's allergic to shellfish." Or, "The thing was, was that I had pneumonia." Note that the verb "is" shows up twice. I've heard several people use this type of phrase and I use it often in conversation.

Why do we do this? Isn't it technically ungrammatical? Anyone? Bueller?