After unwrapping and triangulating model in Blender I export it as FBX for baking. Both in Marmoset and Substance painter I got overlapping issues. Map is different, as you can see in Blender it's ok without overlapping, but after baking I see that it's full of overlaps on baked map in Marmoset. Normals are ok, it's triangulated, no double vertices.
To me, it looks like the issue is both that your UV islands are way too close from each other, and your baking software uses a margin way too big.
For the margin (sometimes called padding), a nice rule of thumb is to divide your model's biggest map resolution by 128.
See more one Edge padding - polycount
When it comes to UVs: they have no resolution, so the margin is expressed by a factor of the total UV space. And since we want a distance of a factor of 128 and each island has its own margin, what we have to use as minimum value is 0.064. You can go higher if you want, historically the default value was 0.16 which is a bit overboard for small resolutions but works for everything.
You can set the margin in either the Unwrap operator or Pack Islands operator options:
In whatever baking software you use, you will have to adapt the baking margin to your model's UVs. In Blender, it's conveniently placed under Baking > Output > Margin:
As for Substance or Marmoset, that's out of this website's scope, but they surely have either a "margin" or "padding" option somewhere.
I know Substance Painter has a "diffuse" option where it progressively fills the empty space between UVs with a gradient based on the margin. That is also a nice added bonus.
