Extrude creates weird mesh

I encounter a strange problem when extruding a simple form.
I made a screenrecording. Is this because I select multiple faces that are not connected?
I tried to remove doubles (zero found), recalculate normals, extrude faces along normals. None of these seem to help. Why is this creating a mess?

do you have any modifiers on the mesh?

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looks like a solidify modifier

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Like Daniel says, I had the solidify modifier enabled. Disabling it solved the problem. Kind of stupid of me not to check the modifiers. Thank you so much for your help, Dan and Daniel.