シートがあるか調べる関数を作ってみました。
Public Function SheetExists(name As String) As Boolean
Dim x As Worksheet
SheetExists = False
For Each x In Sheets
If StrComp(x.name, name, vbTextCompare) = 0 Then SheetExists = True: Exit Function
Next
End Function
使い方
If Not SheetExists("test") Then
Sheets.Add(after:=Sheets(Sheets.Count)).name = "test"
End If