pacman error: FILENAME exists in filesystem

error: failed to commit transaction (conflicting files)
libname: /insert/file/name/here exists in filesystem
Errors occurred, no packages were upgraded.

Why this is happening:

pacman has detected a file conflict, and by design, will not overwrite files for you. This is a design feature, not a flaw.
It is the user’s responsibility to maintain their system, not the package manager’s. (pacman -Qo may be invoked to query which package owns the file, if any.)

What can be done to solve this:

The issue is usually trivial to solve. A safe way is to firstly check if another package owns the file (pacman -Qo <full file path>). If the file is owned by another package file a bug report If the file is not owned by another package rename (or remove) the the file which ‘exists in filesystem’, and reissue the pacman -Syu command. If all goes well, the file may then be removed.

See also: ArchLinux Forum.