Prevent Imps with duplicate IDs (prebid#703)

* Prevent Imps with duplicate IDs.

* Added an initial capacity to the map.

endpoints/openrtb2/auction.go

@@ -258,8 +258,13 @@ func (deps *endpointDeps) validateRequest(req *openrtb.BidRequest) []error {
 		}
 	}
 
+	impIDs := make(map[string]int, len(req.Imp))
 	for index := range req.Imp {
 		imp := &req.Imp[index]
+		if firstIndex, ok := impIDs[imp.ID]; ok {
+			errL = append(errL, fmt.Errorf(`request.imp[%d].id and request.imp[%d].id are both "%s". Imp IDs must be unique.`, firstIndex, index, imp.ID))
+		}
+		impIDs[imp.ID] = index
 		errs := deps.validateImp(imp, aliases, index)
 		if len(errs) > 0 {
 			errL = append(errL, errs...)

endpoints/openrtb2/sample-requests/invalid-whole/imp-id-duplicates.json

@@ -0,0 +1,44 @@
+{
+  "message": "Invalid request: request.imp[0].id and request.imp[1].id are both \"some-impression-id\". Imp IDs must be unique.\n",
+  "requestPayload": {
+    "id": "some-request-id",
+    "site": {
+      "page": "prebid.org"
+    },
+    "imp": [
+      {
+        "id": "some-impression-id",
+        "banner": {
+          "format": [
+            {
+              "w": 300,
+              "h": 250
+            }
+          ]
+        },
+        "ext": {
+          "appnexus": {
+            "placementId": 10433394
+          }
+        }
+      },
+      {
+        "id": "some-impression-id",
+        "banner": {
+          "format": [
+            {
+              "w": 200,
+              "h": 250
+            }
+          ]
+        },
+        "ext": {
+          "appnexus": {
+            "placementId": 10433395
+          }
+        }
+      }
+    ],
+    "tmax": 500
+  }
+}