Index: openjpa-jdbc/src/main/java/org/apache/openjpa/jdbc/sql/SelectImpl.java
===================================================================
--- openjpa-jdbc/src/main/java/org/apache/openjpa/jdbc/sql/SelectImpl.java	(revision 564450)
+++ openjpa-jdbc/src/main/java/org/apache/openjpa/jdbc/sql/SelectImpl.java	(working copy)
@@ -37,6 +37,7 @@
 import java.util.Set;
 import java.util.SortedMap;
 import java.util.Stack;
+import java.util.StringTokenizer;
 import java.util.TreeMap;
 
 import org.apache.commons.collections.iterators.EmptyIterator;
@@ -1369,13 +1370,28 @@
         groupBy(sql, (Joins) null);
     }
 
+    private boolean columnInGrouping(String col) {
+        if (_grouping == null)
+            return false;
+        
+        StringTokenizer st = new StringTokenizer(_grouping.getSQL(),
+                " ,\t\n\r\f");
+        while (st.hasMoreTokens()) {
+            if (StringUtils.equals(st.nextToken(), col))
+                return true;
+        }
+        return false;
+    }
+    
     public void groupBy(SQLBuffer sql, Joins joins) {
         getJoins(joins, true);
-        if (_grouping == null)
-            _grouping = new SQLBuffer(_dict);
-        else
-            _grouping.append(", ");
-        _grouping.append(sql);
+        if (!columnInGrouping(sql.getSQL())) {
+            if (_grouping == null)
+                _grouping = new SQLBuffer(_dict);
+            else
+                _grouping.append(", ");
+            _grouping.append(sql.getSQL());
+        }
     }
 
     public void groupBy(String sql) {
@@ -1384,11 +1400,13 @@
 
     public void groupBy(String sql, Joins joins) {
         getJoins(joins, true);
-        if (_grouping == null)
-            _grouping = new SQLBuffer(_dict);
-        else
-            _grouping.append(", ");
-        _grouping.append(sql);
+        if (!columnInGrouping(sql)) {
+            if (_grouping == null)
+                _grouping = new SQLBuffer(_dict);
+            else
+                _grouping.append(", ");
+            _grouping.append(sql);
+        }
     }
 
     public void groupBy(Column col) {
@@ -1396,13 +1414,14 @@
     }
 
     public void groupBy(Column col, Joins joins) {
-        if (_grouping == null)
-            _grouping = new SQLBuffer(_dict);
-        else
-            _grouping.append(", ");
-
         PathJoins pj = getJoins(joins, true);
-        _grouping.append(getColumnAlias(col, pj));
+        if (!columnInGrouping(getColumnAlias(col, pj))) {
+            if (_grouping == null)
+                _grouping = new SQLBuffer(_dict);
+            else
+                _grouping.append(", ");
+            _grouping.append(getColumnAlias(col, pj));
+        }
     }
 
     public void groupBy(Column[] cols) {
@@ -1410,16 +1429,15 @@
     }
 
     public void groupBy(Column[] cols, Joins joins) {
-        if (_grouping == null)
-            _grouping = new SQLBuffer(_dict);
-        else
-            _grouping.append(", ");
-
         PathJoins pj = getJoins(joins, true);
         for (int i = 0; i < cols.length; i++) {
-            if (i > 0)
-                _grouping.append(", ");
-            _grouping.append(getColumnAlias(cols[i], pj));
+            if (!columnInGrouping(getColumnAlias(cols[i], pj))) {
+                if (_grouping == null)
+                    _grouping = new SQLBuffer(_dict);
+                else
+                    _grouping.append(", ");
+                _grouping.append(getColumnAlias(cols[i], pj));
+            }
         }
     }