Fix double-word typos

Discussion: https://postgr.es/m/20190612184527.GA24266@alvherre.pgsql
Reviewed-by: Michaël Paquier

src/include/executor/tuptable.h

@@ -59,7 +59,7 @@
  * also that a virtual tuple does not have any "system columns".
  *
  * The Datum/isnull arrays of a TupleTableSlot serve double duty.  For virtual
- * slots they they are the authoritative data.  For the other builtin slots,
+ * slots they are the authoritative data.  For the other builtin slots,
  * the arrays contain data extracted from the tuple.  (In this state, any
  * pass-by-reference Datums point into the physical tuple.)  The extracted
  * information is built "lazily", ie, only as needed.  This serves to avoid

src/include/fmgr.h

@@ -96,7 +96,7 @@ typedef struct FunctionCallInfoBaseData
 } FunctionCallInfoBaseData;
 
 /*
- * Space needed for for a FunctionCallInfoBaseData struct with sufficient space
+ * Space needed for a FunctionCallInfoBaseData struct with sufficient space
  * for `nargs` arguments.
  */
 #define SizeForFunctionCallInfo(nargs) \

src/include/utils/snapshot.h

@@ -69,7 +69,7 @@ typedef enum SnapshotType
 	SNAPSHOT_ANY,
 
 	/*
-	 * A tuple is visible iff the tuple tuple is valid as a TOAST row.
+	 * A tuple is visible iff the tuple is valid as a TOAST row.
 	 */
 	SNAPSHOT_TOAST,