fish_add_path: Also deduplicate the new paths

Previously, running `fish_add_path /foo /foo` would result in /foo
being added to $PATH twice.

Now we check that it hasn't already been given, so we skip the
second (and any further) occurence.

share/functions/fish_add_path.fish

@@ -58,11 +58,11 @@ function fish_add_path --description "Add paths to the PATH"
 
         if set -l ind (contains -i -- $p $$var)
             # In move-mode, we remove it from its current position and add it back.
-            if set -q _flag_move
+            if set -q _flag_move; and not contains -- $p $newpaths
                 set -a indexes $ind
                 set -a newpaths $p
             end
-        else
+        else if not contains -- $p $newpaths
             # Without move, we only add it if it's not in.
             set -a newpaths $p
         end

tests/checks/fish_add_path.fish

@@ -58,4 +58,7 @@ or echo "PATH CHANGED!!!" >&2
 PATH=$tmpdir/{bin,etc,link,sbin} fish_add_path -nPpm $tmpdir/{link,sbin} | string replace -a $tmpdir ''
 # CHECK: set -g PATH /link /sbin /bin /etc
 
+# See that trying to add a path twice doesn't duplicate it
+PATH=$tmpdir/{bin,etc,link,sbin} fish_add_path -nPpm $tmpdir/sbin{,} | string replace -a $tmpdir ''
+# CHECK: set -g PATH /sbin /link /bin /etc
 exit 0