diff --git a/db.go b/db.go
index b4c1d33..bf50e40 100644
--- a/db.go
+++ b/db.go
@@ -116,11 +116,15 @@ CREATE TABLE IF NOT EXISTS "relationships" (
 	"to_person_id" INTEGER,
 	"to_item_id" INTEGER,
 	"directed" BOOLEAN, -- if false, the edge goes both ways
- 	"label" TEXT,
+ 	"label" TEXT NOT NULL,
 	FOREIGN KEY ("from_item_id") REFERENCES "items"("id") ON DELETE CASCADE,
 	FOREIGN KEY ("to_item_id") REFERENCES "items"("id") ON DELETE CASCADE,
 	FOREIGN KEY ("from_person_id") REFERENCES "persons"("id") ON DELETE CASCADE,
-	FOREIGN KEY ("to_person_id") REFERENCES "persons"("id") ON DELETE CASCADE
+	FOREIGN KEY ("to_person_id") REFERENCES "persons"("id") ON DELETE CASCADE,
+	UNIQUE ("from_item_id", "to_item_id", "label"),
+	UNIQUE ("from_person_id", "to_person_id", "label"),
+	UNIQUE ("from_item_id", "to_person_id", "label"),
+	UNIQUE ("from_person_id", "to_item_id", "label")
 );
 
 CREATE TABLE IF NOT EXISTS "collections" (
diff --git a/processing.go b/processing.go
index 01ee88f..01f6b39 100644
--- a/processing.go
+++ b/processing.go
@@ -109,7 +109,7 @@ func (wc *WrappedClient) processItemGraph(ig *ItemGraph, state *recursiveState)
 
 				// insert relations to this connected node into DB
 				for _, rel := range relations {
-					_, err = wc.tl.db.Exec(`INSERT INTO relationships
+					_, err = wc.tl.db.Exec(`INSERT OR IGNORE INTO relationships
 					(from_item_id, to_item_id, directed, label)
 					VALUES (?, ?, ?, ?)`,
 						igRowID, connectedIGRowID, !rel.Bidirectional, rel.Label)
@@ -156,7 +156,7 @@ func (wc *WrappedClient) processItemGraph(ig *ItemGraph, state *recursiveState)
 		}
 
 		// store the relation
-		_, err = wc.tl.db.Exec(`INSERT INTO relationships
+		_, err = wc.tl.db.Exec(`INSERT OR IGNORE INTO relationships
 					(from_item_id, to_item_id, directed, label)
 					VALUES (?, ?, ?, ?)`,
 			fromItemRowID, toItemRowID, rr.Bidirectional, rr.Label)