I think they might have had a clash and a showdown running at the same time once. What I can never recall happening is having a clash 2 weeks in a row.
It happened once a couple of years ago when they missed a week, but that is the only time I can think of in the last 6 years.
But, in my opinion, WGT shouldn't even think about another clash until they fix the match clock timer problem. Clash participation in our club was down last week, and I am sure that had a lot to do with it.