Two wagons are placed when tracks are adjacent if the mouse is moved even slightly when clicking. See video: https://youtu.be/E8gc2vsHS_I
I think it's because the mouse is technically also above the track obscured by the wagon so it places a wagon on both tracks but I might be wrong.
Edit: should point out I'm running factorio 1.0.0 with no mods on windows 10.
[1.0.0] Train placement bug (places two wagons instead of one)
Re: [1.0.0] Train placement bug (places two wagons instead of one)
For other developers: this is due to trains snapping-to-a-valid-location-when-built and the snapping being so far that it moves to a completely different set of rails.
If you want to get ahold of me I'm almost always on Discord.
Re: [1.0.0] Train placement bug (places two wagons instead of one)
Thanks for the report. Issue is now fixed for 1.1.