I've tested this issue with thoses setup:

Umbraco 7.3.5 / Courier 2.51.4 Umbraco 7.4.0 / Courier 2.52.0 Umbraco 7.4.0 / Courier 2.52.3

I have the problem on my "main" environment but I'm able to reproduce the same problem with a "clean" install without customization

Using SQL Server database

To reproduce the bug:

1 - Create two member groups (Ex: G1 and G2) 2 - On any item in a site, set public access (role base) and assign G1 and G2 3 - Create a rev on the item with (add + dependencies) 4 - Error creating revision... 5 - Remove 1 of the two member group for the item 6 - Create a new rev on the item with (add+dependencies) 7 - It works.

Here's the log info when there's an error on the rev:

2016-03-01 14:36:43,988 [18] ERROR Umbraco.Courier.Core.TaskManager - [Thread 49] Error PackagingTask with id: eafc9a0f-da26-4ca5-9e8a-92c14f7b10d9 System.InvalidOperationException: La séquence contient plusieurs éléments correspondants. à System.Linq.Enumerable.Single[TSource](IEnumerable1 source, Func2 predicate) à Umbraco.Courier.ItemProviders.AccessProvider.HandlePack(ItemIdentifier id) à Umbraco.Courier.Core.ItemProvider.Package(ItemIdentifier id) à Umbraco.Courier.RepositoryProviders.Local.Package(ItemIdentifier itemId) à Umbraco.Courier.Core.Packaging.RevisionPackaging.PackageItem(ItemIdentifier itemId, QueuedItemIdentifier itemInQueue) à Umbraco.Courier.Core.Packaging.RevisionPackaging.PackageBatch() à Umbraco.Courier.Core.Packaging.RevisionPackaging.PackageBatch() à Umbraco.Courier.Core.Packaging.RevisionPackaging.Package() à Umbraco.Courier.Core.Tasks.PackagingTask.Run() à Umbraco.Courier.Core.BackgroundTaskRunner1.ConsumeTaskInternal(T task) 2016-03-01 14:36:43,988 [18] ERROR Umbraco.Courier.Core.BackgroundTaskRunner - [Thread 49] An error occurred consuming task System.InvalidOperationException: La séquence contient plusieurs éléments correspondants. à System.Linq.Enumerable.Single[TSource](IEnumerable1 source, Func2 predicate) à Umbraco.Courier.ItemProviders.AccessProvider.HandlePack(ItemIdentifier id) à Umbraco.Courier.Core.ItemProvider.Package(ItemIdentifier id) à Umbraco.Courier.RepositoryProviders.Local.Package(ItemIdentifier itemId) à Umbraco.Courier.Core.Packaging.RevisionPackaging.PackageItem(ItemIdentifier itemId, QueuedItemIdentifier itemInQueue) à Umbraco.Courier.Core.Packaging.RevisionPackaging.PackageBatch() à Umbraco.Courier.Core.Packaging.RevisionPackaging.PackageBatch() à Umbraco.Courier.Core.Packaging.RevisionPackaging.Package() à Umbraco.Courier.Core.Tasks.PackagingTask.Run() à Umbraco.Courier.Core.BackgroundTaskRunner1.ConsumeTaskInternal(T task)