The message means your lightmap uvs are overlapping. I am guessing you are using the primary channel for lightmapping. You can solve that issue by creating a secondary uv set right inside unreal.
To do this open the mesh inside unreal and then go to Window > Generate Unique Uvs. the default settings usually work well for this, so hit apply (you can tweak it if you wish).
To view the generated uvs on the top you should see “UV Channel 0” change that to UV Channel 1 and click on the UV button that is next to “Additional Data”.
If you are happy with the layout then go to the details panel and search for “Lightmap”.
Change the “lightmap coordinate index” to 1 and the you can fiddle around with the lightmap resoulution.
Rebake the lighting in your would and that should solve your problem.
Alternatively you can generate a secondary UV set in maya and then open the mesh inside unreal follow step 4 and 5 after you reimport your mesh.
Hope it helps. Sorry for the weird formatting, was just about to leave work. If you need more clarification let me know.