diff --git a/Foxnouns.Backend/Database/DatabaseContext.cs b/Foxnouns.Backend/Database/DatabaseContext.cs
index e6dc524..dae8b28 100644
--- a/Foxnouns.Backend/Database/DatabaseContext.cs
+++ b/Foxnouns.Backend/Database/DatabaseContext.cs
@@ -71,6 +71,22 @@ public class DatabaseContext(DbContextOptions options) : DbContext(options)
         modelBuilder.Entity<Member>().HasIndex(m => new { m.UserId, m.Name }).IsUnique();
         modelBuilder.Entity<Member>().HasIndex(m => m.Sid).IsUnique();
         modelBuilder.Entity<TemporaryKey>().HasIndex(k => k.Key).IsUnique();
+        modelBuilder
+            .Entity<AuthMethod>()
+            .HasIndex(m => new
+            {
+                m.AuthType,
+                m.RemoteId,
+                m.FediverseApplicationId,
+            })
+            .HasFilter("fediverse_application_id IS NOT NULL")
+            .IsUnique();
+
+        modelBuilder
+            .Entity<AuthMethod>()
+            .HasIndex(m => new { m.AuthType, m.RemoteId })
+            .HasFilter("fediverse_application_id IS NULL")
+            .IsUnique();
 
         modelBuilder.Entity<User>().Property(u => u.Sid).HasDefaultValueSql("find_free_user_sid()");
         modelBuilder.Entity<User>().Property(u => u.Fields).HasColumnType("jsonb");
diff --git a/Foxnouns.Backend/Database/Migrations/20241128202508_AddAuthMethodUniqueIndex.cs b/Foxnouns.Backend/Database/Migrations/20241128202508_AddAuthMethodUniqueIndex.cs
new file mode 100644
index 0000000..f6a00b5
--- /dev/null
+++ b/Foxnouns.Backend/Database/Migrations/20241128202508_AddAuthMethodUniqueIndex.cs
@@ -0,0 +1,47 @@
+using Microsoft.EntityFrameworkCore.Infrastructure;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace Foxnouns.Backend.Database.Migrations
+{
+    /// <inheritdoc />
+    [DbContext(typeof(DatabaseContext))]
+    [Migration("20241128202508_AddAuthMethodUniqueIndex")]
+    public partial class AddAuthMethodUniqueIndex : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.CreateIndex(
+                name: "ix_auth_methods_auth_type_remote_id",
+                table: "auth_methods",
+                columns: new[] { "auth_type", "remote_id" },
+                unique: true,
+                filter: "fediverse_application_id IS NULL"
+            );
+
+            migrationBuilder.CreateIndex(
+                name: "ix_auth_methods_auth_type_remote_id_fediverse_application_id",
+                table: "auth_methods",
+                columns: new[] { "auth_type", "remote_id", "fediverse_application_id" },
+                unique: true,
+                filter: "fediverse_application_id IS NOT NULL"
+            );
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropIndex(
+                name: "ix_auth_methods_auth_type_remote_id",
+                table: "auth_methods"
+            );
+
+            migrationBuilder.DropIndex(
+                name: "ix_auth_methods_auth_type_remote_id_fediverse_application_id",
+                table: "auth_methods"
+            );
+        }
+    }
+}
diff --git a/Foxnouns.Backend/Database/Migrations/DatabaseContextModelSnapshot.cs b/Foxnouns.Backend/Database/Migrations/DatabaseContextModelSnapshot.cs
index d012fe0..f9f1609 100644
--- a/Foxnouns.Backend/Database/Migrations/DatabaseContextModelSnapshot.cs
+++ b/Foxnouns.Backend/Database/Migrations/DatabaseContextModelSnapshot.cs
@@ -98,6 +98,16 @@ namespace Foxnouns.Backend.Database.Migrations
                     b.HasIndex("UserId")
                         .HasDatabaseName("ix_auth_methods_user_id");
 
+                    b.HasIndex("AuthType", "RemoteId")
+                        .IsUnique()
+                        .HasDatabaseName("ix_auth_methods_auth_type_remote_id")
+                        .HasFilter("fediverse_application_id IS NULL");
+
+                    b.HasIndex("AuthType", "RemoteId", "FediverseApplicationId")
+                        .IsUnique()
+                        .HasDatabaseName("ix_auth_methods_auth_type_remote_id_fediverse_application_id")
+                        .HasFilter("fediverse_application_id IS NOT NULL");
+
                     b.ToTable("auth_methods", (string)null);
                 });