mount list grows endlessly on automount / autofs setup
The solution is to use /etc/fstab with the net option.
1 - Uncomment this line in /etc/auto_master:
/Network/Servers        -fstab

2 - Fill in fstab as root. For example:
server:/path ignoredmountpoint url net,noowners,url==smb://user:passwd@server/path
...

3 - Run sudo automount -vc
This way, everything is mounted in /Network/Servers automatically on demand and I don't observe multiple mount points anymore.
I think the net option is what makes this work, because I had already tried using fstab before with the same result as an autofs map (i.e. multiple mounts per mountpoint).