I have 2 places to put air filters on my home system. One is a 16x20x1 at the wall intake and the other is 4ft up the same duct at the unit (16x25x1). The filter in the wall is a pain in the ass to change in comparison to the one at the unit. But, they are both usually dirty at 3 months so I have continued to replace both. The repair man that came out recently told me I only needed one filter, either the wall or at the unit. Is this correct? I am worried about the trash that has been collecting on the second filter up line from the first just going into the unit. Or, could 2 filters be placed against each other at the unit or would this be extra strain on the unit having them directly up against one another?