Benelli Supernova double feed with tube extension

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I just picked up a used Benelli Supernova. This is a new system to me, I'm only familiar with my much more simple Mossberg 500. I bought an admittedly cheap tube extension and I'm getting double feeds when trying to run the 2 1/2 inch shells I normally use. They do seem to run fine with the original tube. With the cheap tube extension there is a lot of extra tube space, I can almost fit 3 shells instead of the 2 it is for. If I put in a spacer to tighten it up do you think that may help? Do I just need to buy a better tube extension? Again, they do seem to run fine in the original tube. Thanks.