I have been noticing recently that changing the metre to 4/4 causes a double bar line to appear where the 4/4 begins. Any thoughts on what might be causing this?
If you want to get rid of a double barline, you should delete it, not change it to a single. Changing it means you’ve now got an explicit single barline on top of the default single barline. If you select that single barline now and delete it, the signpost will disappear (and you’ll be left with the default barline).
If you added that 4/4 from the “Used in This Flow” section of the right panel in Write mode, and had previously created a 4/4 that coincided with a double barline, then that might explain why the double barline appears here, too.
Barlines in Dorico “come from” time signatures, because it’s the time signature that defines where barlines should appear (after 3, 4, 5 beats etc). Therefore, time signatures can become associated with barline overrides, and that association is preserved in the “Used in This Flow” section. If you input a fresh time signature from scratch, it won’t carry that association. (We appreciate that this associated-barline-type isn’t obvious from the panel alone.)