Yep, definitely a stuck line in the video address circuitry somewhere. Depending on which line it is, you could get any pattern of doubling from 8-pixel vertical stripes (bit 0) to two 256-pixel vertical stripes (bit 5) to horizontal stripes of any number of lines (bits 7+).
My Plus logic board schematic is hard to read, but it looks like you want to probe signal VA5 (video address bit 5), which goes from U1F pin 10 to U3G pin 13. Also check VA4 (U1F pin 11, U3G pin 3) and VA6 (U1F pin 9, U4G pin 3). I *think* the fault would be on bit 5, but if you compare what's happening on those different lines you might see if anything looks different between them. A scope could help.