SketchUp doesn’t work very well with two end-points that are within about 0.001 inch of each other. (It doesn’t matter if those two points are directly connected with a single edge, or whether the two points are the ends of two separate edges. Either situation causes trouble.)
The most-often recommended technique to work around this issue with SketchUp is to utilize the so-called “Dave” method (popularized by @DaveR): Place all the relevant geometry into a component. Make a copy of the component off to the side. Scale up that copy by 100X or 1000X (I use 100X). Edit the big copy. The small copy will follow suit. When done with modifications, you can simply delete the big copy with no worry about scaling it back down, jostling its position slightly, etc.
Even when scaled up in such a manner it’s still possible to create very closely-spaced end points, and thus encounter the same problem. But it’s 100X or 1000X less likely, so to speak, that you’ll encounter it in the big version. When I do encounter it, I manually fudge the geometry to share a common point or to separate the two points a bit further, being reasonably satisfied that the fudge will be extremely tiny in the natural-size version.