Visual Basic problems in Alphabets

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VB

I have a textbox2.text and I declare this as a string namely myletters
and I also have a label36.text and i also declare this as a string namely patterns

I will only input on the textbox is letters from "a" to "z" only and the reason why there is a label36 is that when i put the same letter again it will show an error message.

This is my code so far.
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Imports System.Text.RegularExpressions

Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button3.Click
        Dim patterns As String
        patterns = Label36.Text
        Dim myletters As String
        myletters = TextBox2.Text
        Dim matches As MatchCollection = Regex.Matches(myletters, patterns)
        If Regex.IsMatch(patterns, myletters) Then
            MsgBox("You already typed this letter.")
        Else
            Label36.Text = Label36.Text + myletters + ","
            End If
    End Sub

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What use on the top is my code so far that will show error if i already put a same letter again and give this if i dont put the same letters.


Example Output:

Set A: a,b,c,d,e


Now here is my questions.
1.How can i input my first letter without following a comma?
Example:
a

But my output is
Example:
a,
2.How can i set my textbox to the maximum of input of just 1 letter?
3.How can i put commas in right time and format?
Example 1:
a
Example 2:
a,b
But my example is
Example 1:
a,
Example 2:
a,b,
4.How can i arrange the alphabet letters in the right positions?
Example 1:
a,z
If i will input "x" the output must be
Example 2:
a,x,z



Thank you, any answer will be accepted TY.

[edit]SHOUTING removed - OriginalGriff[/edit]