REF: ensure to apply suffixes before concat step in merge code (#48416)

Author: jorisvandenbossche

pandas/core/reshape/merge.py

@@ -764,8 +764,9 @@ def _reindex_and_concat(
 
         from pandas import concat
 
+        left.columns = llabels
+        right.columns = rlabels
         result = concat([left, right], axis=1, copy=copy)
-        result.columns = llabels.append(rlabels)
         return result
 
     def get_result(self, copy: bool = True) -> DataFrame: