dotnet/efcore · error · OperationException

The name '{migrationName}' is used by an existing migration.

Error message

The name '{migrationName}' is used by an existing migration.

What it means

Before scaffolding, `MigrationsScaffolder.ScaffoldMigration` calls `MigrationsAssembly.FindMigrationId(migrationName)`; if a migration with that name already exists in the configured migrations assembly it throws `OperationException(DuplicateMigrationName)`. EF requires migration names to be unique within an assembly so that `GetId()` lookups and `__EFMigrationsHistory` entries stay unambiguous.

Source

Thrown at src/EFCore.Design/Migrations/Design/MigrationsScaffolder.cs:80

    /// </param>
    /// <param name="language">The project's language.</param>
    /// <param name="dryRun">If <see langword="true" />, then nothing is actually written to disk.</param>
    /// <returns>The scaffolded migration.</returns>
    public virtual ScaffoldedMigration ScaffoldMigration(
        string migrationName,
        string? rootNamespace,
        string? subNamespace = null,
        string? language = null,
        bool dryRun = false)
    {
        if (string.Equals(migrationName, "migration", StringComparison.OrdinalIgnoreCase))
        {
            throw new OperationException(DesignStrings.CircularBaseClassDependency);
        }

        if (Dependencies.MigrationsAssembly.FindMigrationId(migrationName) != null)
        {
            throw new OperationException(DesignStrings.DuplicateMigrationName(migrationName));
        }

        var overrideNamespace = rootNamespace == null;
        var subNamespaceDefaulted = false;
        if (string.IsNullOrEmpty(subNamespace) && !overrideNamespace)
        {
            subNamespaceDefaulted = true;
            subNamespace = "Migrations";
        }

        var (key, typeInfo) = Dependencies.MigrationsAssembly.Migrations.LastOrDefault();

        var migrationNamespace =
            (!string.IsNullOrEmpty(rootNamespace)
                && !string.IsNullOrEmpty(subNamespace))
                ? rootNamespace + "." + subNamespace
                : !string.IsNullOrEmpty(rootNamespace)
                    ? rootNamespace

View on GitHub (pinned to dbf9771522)

Solutions

Pick a new, descriptive name for the migration.
If the existing one is stale/unwanted, remove it first with `dotnet ef migrations remove` (only if unapplied) or delete the files manually and regenerate.
If two branches collided, rename one migration's class, file, and timestamp prefix, or re-add with a distinct name.

Example fix

// before
dotnet ef migrations add InitialCreate   // already exists
// after
dotnet ef migrations add AddUsersTable

Defensive patterns

Strategy: validation

Validate before calling

// Check the migrations assembly for an existing name before scaffolding.
string migrationName = /* from user */;
var existing = ((Microsoft.EntityFrameworkCore.Migrations.IMigrationsAssembly)
    serviceProvider.GetRequiredService<Microsoft.EntityFrameworkCore.Migrations.IMigrationsAssembly>())
    .FindMigrationId(migrationName);
if (existing is not null)
    throw new InvalidOperationException($"A migration named '{migrationName}' already exists ({existing}).");

scaffolder.ScaffoldMigration(migrationName, rootNamespace, subNamespace);

Prevention

List existing migrations (`dotnet ef migrations list`) before adding a new one.
In CI, generate names with a timestamp/feature token to avoid collisions across branches.
When merging branches, reconcile any same-named migrations before continuing.

When it happens

Trigger: `dotnet ef migrations add InitialCreate` when an `InitialCreate` migration class already exists in the migrations assembly; calling `ScaffoldMigration` with a name already produced by `MigrationsIdGenerator.GenerateId`.

Common situations: Re-running a command after a partial/failed previous run left the class behind, merging branches that each added a same-named migration, or forgetting a migration was already added on another machine.

Related errors

AI-assisted analysis of dotnet/efcore@dbf9771522 (2026-08-06). Data as JSON: /api/errors/55ba42d08df01561. Report an issue: GitHub.