What doesn't work about it?

Uplgrading from 1.10 to 1.12 gives the following error

A duplicate value cannot be inserted into a unique index. [ Table name = merchTypeField,Constraint name = PK_merchTypeField ]

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Data.SqlServerCe.SqlCeException: A duplicate value cannot be inserted into a unique index. [ Table name = merchTypeField,Constraint name = PK_merchTypeField ]

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:

[SqlCeException (0x80004005): A duplicate value cannot be inserted into a unique index. [ Table name = merchTypeField,Constraint name = PKmerchTypeField ]] System.Data.SqlServerCe.SqlCeCommand.ProcessResults(Int32 hr) +261 System.Data.SqlServerCe.SqlCeCommand.ExecuteCommandText(IntPtr& pCursor, Boolean& isBaseTableCursor) +670 System.Data.SqlServerCe.SqlCeCommand.ExecuteCommand(CommandBehavior behavior, String method, ResultSetOptions options) +474 System.Data.SqlServerCe.SqlCeCommand.ExecuteNonQuery() +44 StackExchange.Profiling.Data.ProfiledDbCommand.ExecuteNonQuery() in c:\Code\github\SamSaffron\MiniProfiler\StackExchange.Profiling\Data\ProfiledDbCommand.cs:266 Umbraco.Core.Persistence.<>cDisplayClass1.0() in d:\Dev\Umbraco-CMS\src\Umbraco.Core\Persistence\PetaPocoCommandExtensions.cs:56 Umbraco.Core.Persistence.FaultHandling.RetryPolicy.ExecuteAction(Func1 func) in d:\Dev\Umbraco-CMS\src\Umbraco.Core\Persistence\FaultHandling\RetryPolicy.cs:172 Umbraco.Core.Persistence.Database.Insert(String tableName, String primaryKeyName, Boolean autoIncrement, Object poco) in d:\Dev\Umbraco-CMS\src\Umbraco.Core\Persistence\PetaPoco.cs:1370 Umbraco.Core.Persistence.Database.Insert(String tableName, String primaryKeyName, Object poco) in d:\Dev\Umbraco-CMS\src\Umbraco.Core\Persistence\PetaPoco.cs:1215 Merchello.Core.Persistence.Migrations.Upgrades.TargetVersionOneElevenZero.AddEntityCollectionTypeField.Up() in c:\Working Repositories\GitHub\Merchello\src\Merchello.Core\Persistence\Migrations\Upgrades\TargetVersionOneElevenZero\AddEntityCollectionTypeField.cs:38 Umbraco.Core.Persistence.Migrations.MigrationRunner.InitializeMigrations(List1 migrations, Database database, DatabaseProviders databaseProvider, Boolean isUpgrade) in d:\Dev\Umbraco-CMS\src\Umbraco.Core\Persistence\Migrations\MigrationRunner.cs:155 Umbraco.Core.Persistence.Migrations.MigrationRunner.Execute(Database database, DatabaseProviders databaseProvider, Boolean isUpgrade) in d:\Dev\Umbraco-CMS\src\Umbraco.Core\Persistence\Migrations\MigrationRunner.cs:62 Merchello.Core.Persistence.Migrations.CoreMigrationManager.UpgradeMerchello(Database database) in c:\Working Repositories\GitHub\Merchello\src\Merchello.Core\Persistence\Migrations\CoreMigrationManager.cs:157 Merchello.Core.Persistence.Migrations.CoreMigrationManager.EnsureMerchelloVersion() in c:\Working Repositories\GitHub\Merchello\src\Merchello.Core\Persistence\Migrations\CoreMigrationManager.cs:75 Umbraco.Core.CoreBootManager._11(IApplicationEventHandler x) in d:\Dev\Umbraco-CMS\src\Umbraco.Core\CoreBootManager.cs:284 Umbraco.Core.EnumerableExtensions.ForEach(IEnumerable1 items, Action1 action) in d:\Dev\Umbraco-CMS\src\Umbraco.Core\EnumerableExtensions.cs:90 Umbraco.Core.CoreBootManager.Complete(Action1 afterComplete) in d:\Dev\Umbraco-CMS\src\Umbraco.Core\CoreBootManager.cs:274 Umbraco.Web.WebBootManager.Complete(Action1 afterComplete) in d:\Dev\Umbraco-CMS\src\Umbraco.Web\WebBootManager.cs:179 Umbraco.Core.UmbracoApplicationBase.StartApplication(Object sender, EventArgs e) in d:\Dev\Umbraco-CMS\src\Umbraco.Core\UmbracoApplicationBase.cs:59

[HttpException (0x80004005): A duplicate value cannot be inserted into a unique index. [ Table name = merchTypeField,Constraint name = PK_merchTypeField ]] System.Web.HttpApplicationFactory.EnsureAppStartCalledForIntegratedMode(HttpContext context, HttpApplication app) +12639357 System.Web.HttpApplication.RegisterEventSubscriptionsWithIIS(IntPtr appContext, HttpContext context, MethodInfo[] handlers) +175 System.Web.HttpApplication.InitSpecial(HttpApplicationState state, MethodInfo[] handlers, IntPtr appContext, HttpContext context) +304 System.Web.HttpApplicationFactory.GetSpecialApplicationInstance(IntPtr appContext, HttpContext context) +404 System.Web.Hosting.PipelineRuntime.InitializeApplication(IntPtr appContext) +475

[HttpException (0x80004005): A duplicate value cannot be inserted into a unique index. [ Table name = merchTypeField,Constraint name = PK_merchTypeField ]] System.Web.HttpRuntime.FirstRequestInit(HttpContext context) +12656404 System.Web.HttpRuntime.EnsureFirstRequestInit(HttpContext context) +159 System.Web.HttpRuntime.ProcessRequestNotificationPrivate(IIS7WorkerRequest wr, HttpContext context) +12496021

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