[Merkaartor] Error "Precondition failed", duplicate way

[Yves Goergen]

I have joined two roads in the map and uploaded this single change to
the OSM server. The actions were Update and Remove on a road with the
same name.

I got the error message "Precondition failed". I'm sorry I cannot tell
you more, if there was more, because I clicked it away too early.

I quit Merkaartor and downloaded the same region again, and there was no
problem visible at first. The roads are joined. But selecting the entire
region and investigating the single selected objects, one part of the
road was still there, albeit now covered by the other part. So all nodes
were duplicated (?) and the way was extended, but the other way wasn't
removed.

This is a tricky error because there's no obvious problem left, but in
fact a part of the road was there twice.

I've had this error before in other situations (never joined roads
before today).

Merkaartor version: 0.12
Reproducibility: not tested

-- 
Yves Goergen "LonelyPixel" <nospam.list at unclassified.de>
Visit my web laboratory at http://beta.unclassified.de