--- a/pkg/imports/gm.go	Thu Mar 28 16:38:15 2019 +0100
+++ b/pkg/imports/gm.go	Thu Mar 28 16:40:02 2019 +0100
@@ -50,9 +50,9 @@
 FROM waterway.gauges
 WHERE (location).country_code = users.current_user_country()`
 
-	// TODO: Currently this statement updates existing data sets. In case we want
-	// 'historization' we need to develop an other mechanism to keep existing
-	// data.
+	// Note: we do not expect corrections of data through this service.  So
+	// any constraint conflicts are triggered by actual redundat data which
+	// can be dropped.
 	insertGMSQL = `
 INSERT INTO waterway.gauge_measurements (
   fk_gauge_id,
@@ -87,20 +87,7 @@
   $18,
   $19
 )
-ON CONFLICT ON CONSTRAINT gauge_measurements_fk_gauge_id_measure_date_staging_done_key
-DO UPDATE SET
-country_code = EXCLUDED.country_code,
-sender = EXCLUDED.sender,
-language_code = EXCLUDED.language_code,
-date_issue = EXCLUDED.date_issue,
-reference_code= EXCLUDED.reference_code,
-water_level = EXCLUDED.water_level,
-predicted = EXCLUDED.predicted,
-is_waterlevel = EXCLUDED.is_waterlevel,
-value_min = EXCLUDED.value_min,
-value_max = EXCLUDED.value_max,
-date_info = EXCLUDED.date_info,
-source_organization = EXCLUDED.source_organization
+ON CONFLICT DO NOTHING
 RETURNING id
 `
 )
@@ -335,6 +322,8 @@
 			} else {
 				referenceCode = string(*wrm.Reference_code)
 			}
+
+			var newCnt int = 0
 			for _, measure := range wrm.Measure {
 				var unit string
 				if measure.Unit == nil {
@@ -370,12 +359,17 @@
 					msg.Identification.Originator,
 					true, // staging_done
 				).Scan(&gid)
-				if err != nil {
+				switch {
+				case err == sql.ErrNoRows:
+					// thats expected, nothing to do
+				case err != nil:
 					return nil, err
+				default:
+					newCnt++
 				}
 			}
 			feedback.Info("Inserted %d measurements for %s",
-				len(wrm.Measure), curr)
+				newCnt, curr)
 			gids = append(gids, curr)
 		}
 	}

--- a/schema/gemma.sql	Thu Mar 28 16:38:15 2019 +0100
+++ b/schema/gemma.sql	Thu Mar 28 16:40:02 2019 +0100
@@ -338,10 +338,22 @@
         date_info timestamp with time zone NOT NULL DEFAULT CURRENT_TIMESTAMP,
         source_organization varchar NOT NULL, -- "originator"
         staging_done boolean NOT NULL DEFAULT false,
-        -- So we can have a staged and
-        -- a non-staged fk_gauge_id/measure_date pair.
-        UNIQUE (fk_gauge_id, measure_date, staging_done)
     )
+    -- Constraints are conditional for gauge_measurements, as they
+    -- differ between predicted values and measured ones.  PG does not
+    -- have real conditional unique constraints, but we can use unique
+    -- indeces for that.
+    --
+    -- So we can have a staged and a non-staged
+    -- fk_gauge_id/measure_date pairs in measured values.
+    CREATE UNIQUE INDEX gm_measured_unique_constraint
+        ON gauge_measurements (fk_gauge_id, measure_date, staging_done)
+        WHERE NOT predicted;
+    -- And we can have multiple predictions for one point in time
+    -- (but they are never staged).
+    CREATE UNIQUE INDEX gm_predicted_unique_constraint
+        ON gauge_measurements (fk_gauge_id, measure_date, date_issue)
+        WHERE predicted;
 
     CREATE TABLE waterway_axis (
         id int PRIMARY KEY GENERATED BY DEFAULT AS IDENTITY,