diff --git a/lib/OpenLayers/Layer/HTTPRequest.js b/lib/OpenLayers/Layer/HTTPRequest.js
index 53f68126b5..91830c00e0 100644
--- a/lib/OpenLayers/Layer/HTTPRequest.js
+++ b/lib/OpenLayers/Layer/HTTPRequest.js
@@ -121,7 +121,15 @@ OpenLayers.Layer.HTTPRequest.prototype =
         // create a new params hashtable with all the layer params and the 
         // new params together. then convert to string
         var allParams = OpenLayers.Util.extend(new Object(), this.params);
-        var allParams = OpenLayers.Util.extend(allParams, newParams);
+        allParams = OpenLayers.Util.extend(allParams, newParams);
+        // ignore parameters that are already in the url search string
+        var urlParams = OpenLayers.Util.upperCaseObject(
+                            OpenLayers.Util.getArgs(url));
+        for(var key in allParams) {
+            if(key.toUpperCase() in urlParams) {
+                delete allParams[key];
+            }
+        }
         var paramsString = OpenLayers.Util.getParameterString(allParams);
         if (paramsString != "") {
             var lastServerChar = url.charAt(url.length - 1);
diff --git a/tests/test_Layer_HTTPRequest.html b/tests/test_Layer_HTTPRequest.html
index 92221e3c2e..c18a4b3244 100644
--- a/tests/test_Layer_HTTPRequest.html
+++ b/tests/test_Layer_HTTPRequest.html
@@ -89,7 +89,7 @@
         tParams = { layers: 'basic', 
                    format: 'image/png'};
         
-        t.plan( 8 );
+        t.plan( 9 );
 
   // without ?        
         tUrl = "http://octo.metacarta.com/cgi-bin/mapserv";
@@ -109,6 +109,12 @@
         layer = new OpenLayers.Layer.HTTPRequest(name, tUrl, tParams, null);
         str = layer.getFullRequestString();
         t.eq(str, "http://octo.metacarta.com/cgi-bin/mapserv?param1=5&layers=basic&format=image/png", "getFullRequestString() works for url with ?param1=5");
+  
+  // with ?param1=5&
+        tUrl = "http://octo.metacarta.com/cgi-bin/mapserv?param1=5&format=image/jpeg";
+        layer = new OpenLayers.Layer.HTTPRequest(name, tUrl, tParams, null);
+        str = layer.getFullRequestString();
+        t.eq(str, "http://octo.metacarta.com/cgi-bin/mapserv?param1=5&format=image/jpeg&layers=basic", "getFullRequestString() doesn't override already-existing params in URL");
 
  
   // with ?param1=5&
@@ -116,6 +122,8 @@
         layer = new OpenLayers.Layer.HTTPRequest(name, tUrl, tParams, null);
         str = layer.getFullRequestString();
         t.eq(str, "http://octo.metacarta.com/cgi-bin/mapserv?param1=5&layers=basic&format=image/png", "getFullRequestString() works for url with ?param1=5&");
+  
+
 
   // passing in new params
         layer = new OpenLayers.Layer.HTTPRequest(name, tUrl, tParams, null);