Yes, "cp -Rp" will treat ACLs properly and not duplicate like Finder, "cp -R" just ignores them, whatever works... but file a bug if you want this fixed (bugreporter.apple.com)
For those who want to fix their files that have tons of ACL you can kill them all, make a textedit (text only Command-Shift-T), paste this in:
#!/bin/bash
IFS=$'\t\n'
find "${@}" -exec chmod -N {} \;
Open terminal and add the execute bit with: chmod +x <drag file from finder into terminal> <Enter>
Then run with the name of the path as the argument... for more detail
http://hints.macworld.com/article.php?story=20070501065737278
For the troubleshooters:
I got it down to the bare minumum on a clean OS X Server install, not AD bound or anything extraneous. Just AFP running on a server with an IP with forward reverse lookup. I made a share, added admin (501) to the ACL (as Read only), touched a file and duplicated it in Finder, locally on the server.
10.6-10.6.2 functioned fine. 10.6.3 and 10.6.4 added bizarre behavior where a WRITE ACE was added to the ACL, but the ACEs weren't "stacking up", 10.6.5 and 10.6.6 added the redundant ACEs that when you tried copying/duplicating packages and apps with Finder would result in insane amounts of duplication, try Chess.app you'll get 18 ACEs deep in one Finder copy! 3rd copy fails with Error -41
So get reporting folks, 10.6.7 might be the last Snow Leopard update, and even if it isn't I'm getting pretty tired of dealing with this bug and don't want to wait until 10.6.8! So I just filed my bug #9160099 (another poster referenced his bug #8816661, that 343,438 bug reports later!)