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    --- implementation
    public class CustomBehavior : IRegionBehavior
    {
      ...
    }

    --- bootstrapper
    protected override IRegionBehaviorFactory ConfigureDefaultRegionBehaviors()
    {

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namespace Prism.Regions
{
    /// <summary>
    /// Interface for allowing extensible behavior on regions.
    /// </summary>
    public interface IRegionBehavior
    {
        /// <summary>
        /// The region that this behavior is extending.
        /// </summary>

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        public IRegion Initialize(T regionTarget, string regionName)
        {
            if (regionName == null)
                throw new ArgumentNullException(nameof(regionName));

            IRegion region = this.CreateRegion();
            region.Name = regionName;

            SetObservableRegionOnHostingControl(region, regionTarget);

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            // creating duplicate regions via region.add correctly throws exception
             var region = regionManager.Regions["ViewA"];
             region.Add(unityContainer.Resolve<ViewA>());
             region.Add(unityContainer.Resolve<ViewA>());

            // creating duplicate regions via regionManager.AddToRegion does not throw exception
             regionManager.AddToRegion("ViewA", unityContainer.Resolve<ViewA>());
             regionManager.AddToRegion("ViewA", unityContainer.Resolve<ViewA>());

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            // Uncomment the following three lines and we get exception, this gets fixed with scoped region
             var region = regionManager.Regions["ViewA"];
             region.Add(unityContainer.Resolve<ViewA>());
             region.Add(unityContainer.Resolve<ViewA>());

            // the following works
            // regionManager.AddToRegion("ViewA", unityContainer.Resolve<ViewA>());
            // regionManager.AddToRegion("ViewA", unityContainer.Resolve<ViewA>());

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        public IRegionManager AddToRegion(string regionName, object view)
        {
            if (!Regions.ContainsRegionWithName(regionName))
                throw new ArgumentException(string.Format(Thread.CurrentThread.CurrentCulture, Resources.RegionNotFound, regionName), nameof(regionName));

            return Regions[regionName].Add(view);
        }

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--ViewA.xaml
<ItemsControl regions:RegionManager.RegionName="RegionA"></ContentControl>

--ViewB.xaml
<ContentControl regions:RegionManager.RegionName="RegionB"></ItemsControl>

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--ViewA.xaml
<ItemsControl regions:RegionManager.RegionName="RegionA"></ContentControl>

--ViewB.xaml
<ContentControl regions:RegionManager.RegionName="RegionB"></ItemsControl>

--ViewC.xaml
<ContentControl regions:RegionManager.RegionName="RegionC"></ContentControl>

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--ViewA.xaml
<ItemsControl regions:RegionManager.RegionName="RegionA"></ContentControl>

--ViewB.xaml
<ItemsControl regions:RegionManager.RegionName="RegionB"></ItemsControl>

--ViewC.xaml
<ContentControl regions:RegionManager.RegionName="RegionC"></ContentControl>

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--ViewA.xaml
<ContentControl regions:RegionManager.RegionName="RegionA"></ContentControl>

--ViewB.xaml
<ItemsControl regions:RegionManager.RegionName="RegionB"></ItemsControl>

--ViewC.xaml
<ContentControl regions:RegionManager.RegionName="RegionC"></ContentControl>