diff --git a/app/dbtables.py b/app/dbtables.py
index 6d80d57..6c6cbe8 100644
--- a/app/dbtables.py
+++ b/app/dbtables.py
@@ -53,7 +53,7 @@ class NoteColoursTable(Model):
     __tablename__ = "notecolours"
 
     id: Mapped[int] = mapped_column(primary_key=True, autoincrement=True)
-    substring: Mapped[str] = mapped_column(String(256), index=True)
+    substring: Mapped[str] = mapped_column(String(256), index=True, unique=True)
     colour: Mapped[str] = mapped_column(String(21), index=False)
     enabled: Mapped[bool] = mapped_column(default=True, index=True)
     foreground: Mapped[Optional[str]] = mapped_column(String(21), index=False)