SQL 2012 :: Case Statement Causes Duplicates

I have successfully built a messaging system into my application, I now in the process of displaying the messages in the UI.

The following are how my tables are constructed.

CREATE TABLE [MailBox].[Message](
[Id] [bigint] IDENTITY(1,1) NOT NULL,
[SenderId] [bigint] NOT NULL,
[Message] [nvarchar](max) NOT NULL,
[SentDate] [datetime] NOT NULL,
CONSTRAINT [PK_MailBox.Message] PRIMARY KEY CLUSTERED

[Code] .....

Now I haven't set the foreign key on the MessageReceipient table yet. When someone sends me an email I insert a record into [MailBox].[Message] and output the insert id into MessageReceipient along with the ReceipientId this is working as expected, when I then click on my inbox I call the following stored procedure:

Select p.Username, count(mr.RecipientId) [TotalMessages],
CASE
WHEN mr.ReadDate is null then 1 -- New message
WHEN mr.ReadDate is not null then 0 -- Message has been read
END AS NewMessage
FROM [User].[User_Profile] p
JOIN [MailBox].[Message] m on p.Id = m.SenderId
JOIN [MailBox].[MessageRecipient] mr on m.Id = mr.MessageId
GROUP BY p.Username, mr.RecipientId, mr.ReadDate

This will give me the person who has emailed me, the total amount of messages and if the message is new or its been read, I do this by checking the ReadDate column as shown in the case statement (but this gives me duplicates, which is not what I want). Lets say user1 emails me 5 times so when I call this proc I will have the same user displayed to me 5 times, what I'm trying to achieve with the proc is it will show User1 as the following:

User1 5 Messages 1 or 0 New Messages

I can get it to display as follow when I remove the case statement

User1 5 Messages

but as soon as I add the case statement back in then I get 5 rows.

How can I change this proc in such a way that it will display the data as follows;

User1 5 Messages 1 or 0 New Messages

New Messages is dependent on ReadDate if its null then we have a new message, otherwise its been read.