In my project trivial-utf-8 is already loaded. Postmodern does it again which leads to warnings:
If convenient, you can arrange for the other load not to happen, since whichever software is loading it will be able to use the on in Postmodern as well.
For a proper solution, the package name used by postmodern's internal trivial-utf8 (and ieee-floats) should be changed, so that it stops colliding with separately loaded instances of those libraries. If you feel up to this, please submit a patch. If not, I'll get to it eventually.
Best, Marijn