Both the method signature and documentation implies that return value for the method is a list placers created by the current call for the library item given as argument. This is not the case, it actually returns also previously generated placers of other library items if they still exist in the scene.

Excerpt from documentation:

/// <summary>

/// Evaluates, creates and adds the possible placers for a given library item to the currently selected group.

/// </summary>

/// <param name="itemName">Library ID of an item that is being attached.</param>

/// <returns>A collection of created placers.</returns>

/// <seealso cref="GetPlacers(Group, ILibraryItem, string)"/>

ModelItemList<ConfigElement> GetPlacers(string itemName);