diff --git a/pkg/services/ngalert/writer/datasourcewriter.go b/pkg/services/ngalert/writer/datasourcewriter.go
index cc7036974e7..3cb082fb639 100644
--- a/pkg/services/ngalert/writer/datasourcewriter.go
+++ b/pkg/services/ngalert/writer/datasourcewriter.go
@@ -217,6 +217,8 @@ func (w *DatasourceWriter) makeWriter(ctx context.Context, orgID int64, dsUID st
 	}
 
 	for k, values := range dsHeaders {
+		// Clear out any custom writer headers before adding the data souce ones.
+		headers.Del(k)
 		for _, v := range values {
 			headers.Add(k, v)
 		}
diff --git a/pkg/services/ngalert/writer/datasourcewriter_test.go b/pkg/services/ngalert/writer/datasourcewriter_test.go
index 349d8b9191a..04c86a954b3 100644
--- a/pkg/services/ngalert/writer/datasourcewriter_test.go
+++ b/pkg/services/ngalert/writer/datasourcewriter_test.go
@@ -141,7 +141,7 @@ func setupDataSources(t *testing.T) *testDataSources {
 	res.DataSourceHeaders["prom-4"] = http.Header{
 		"X-Scope-OrgID":   []string{"test-user"},
 		"X-Test-Header":   []string{"test-value"},
-		"X-Double-Header": []string{"one", "two", "three"},
+		"X-Double-Header": []string{"one", "two"},
 	}
 
 	return res
@@ -231,7 +231,7 @@ func TestDatasourceWriter(t *testing.T) {
 		cHeaders := map[string]string{
 			"X-Custom-Header":  "test-value",
 			"X-Another-Header": "another-value",
-			overwrittenHeader:  "overwritten", // Data source headers should be overwritten by custom headers.
+			overwrittenHeader:  "should-be-overwritten", // Data source headers overwrite custom headers.
 		}
 
 		cfg = DatasourceWriterConfig{
@@ -249,17 +249,20 @@ func TestDatasourceWriter(t *testing.T) {
 		require.Len(t, dsHeaders, 3)
 
 		// We're confirming we have a data source header with the same name but different value.
-		// This one should not be sent in the request.
 		require.NotEmpty(t, dsHeaders[overwrittenHeader])
 		require.NotEqual(t, dsHeaders[overwrittenHeader], cHeaders[overwrittenHeader])
 
-		// All headers (except for the one that was overwritten) should have been used.
+		// All data source headers should have been used.
 		for k, vv := range dsHeaders {
-			if k != overwrittenHeader {
-				assert.Equal(t, vv, testDS.prom4.LastHeaders.Values(k))
-			}
+			assert.Equal(t, vv, testDS.prom4.LastHeaders.Values(k))
 		}
+
+		// All custom headers except for the overwritten one should have been used.
 		for k, v := range cHeaders {
+			if k == overwrittenHeader {
+				assert.NotEqual(t, v, testDS.prom4.LastHeaders.Get(k))
+				continue
+			}
 			assert.Equal(t, v, testDS.prom4.LastHeaders.Get(k))
 		}
 	})