global: Fix duplicate word typos

Used regex to find these typos:

    (?<!struct )(?<=\s)([a-z]{1,}) \1(?=\s)

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

oidtree.c

@@ -47,7 +47,7 @@ void oidtree_insert(struct oidtree *ot, const struct object_id *oid)
 
 	/*
 	 * n.b. Current callers won't get us duplicates, here.  If a
-	 * future caller causes duplicates, there'll be a a small leak
+	 * future caller causes duplicates, there'll be a small leak
 	 * that won't be freed until oidtree_clear.  Currently it's not
 	 * worth maintaining a free list
 	 */