Devices are listed twice, if PCManFM is not started as daemon (pcmanfm -d)
I'm using PCManFM 1.0.2~alpha1, but this bug appears since the version 1.0.1.
I did some additional fixes in the latest git head.
The problem is caused by new gvfs, but can be avoided by adding some more checks.
That was NOT a fix but another workaround which makes libfm slower. :(
And that issue is reported to gvfs and is already fixed by them:
Just to point out that gvfs 1.14.2 with the fix has been released for some time so folks should rather bother their distros to update it.