keep duplicates out of $PATH on source

add_to_PATH () {
  for d; do
    d=$({ cd -- "$d" && { pwd -P || pwd; } } 2>/dev/null)  # canonicalize symbolic links
    if [ -z "$d" ]; then continue; fi  # skip nonexistent directory
    case ":$PATH:" in
      *":$d:"*) :;;
      *) PATH=$PATH:$d;;
    esac
  done
}
add_to_PATH ~/perl5/bin ~/.bin

The line for symbolic link canonicalization is optional. If you remove it, also remove the next line (if you want to keep nonexistent directories), or change it to

if ! [ -d "$d" ]; then continue; fi

Note that the symlink canonicalization method only guarantees unicity amongst directories that were added by this function. It also doesn't handle edge cases like an NFS directory mounted on two locations or a Linux bind mount.

You could put a test around the "append this directory to path" command which would check to see if foo is already in the path before adding it, but it wouldn't buy you much.

First, the test itself would be costly compared to appending a duplicate element. Secondly, a redundant element later in the path has no effect upon what does get executed when you execute a given command because the first matching executable in the path will still be the one executed. Finally most shells cache prior path hits in a hash table so the second time you execute my_command the path isn't even searched.

About the only thing that not appending redundant entries will get you is a prettier looking path, but most paths are pretty ugly to begin with. If this aesthetic goal is really important to you, tell us which shell you are using and I can conjure up a function to "append this to path only if it isn't present" function.