Open connection error

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VB

I have the following code:
I got the error you try to open a connection which is already open, although I always check if the connection is closed then I open the connection.
The error was in the mycon.open() in the code below.

I want to add somthing my program have different threads, I don't know that if one of the threads open the connection does the other threads can use the database.

What I need to know can two functions use the database simulatinously if each one has it is own connection or not?

Sub get_volume_small(ByVal level As Single, ByRef hieght As List(Of Single), ByRef volume As List(Of Single), ByVal id As Integer)


        Dim mycon As New OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0;Data Source='" + My.Computer.FileSystem.CurrentDirectory + "\gas.accdb';Persist Security Info=True")
        Try
            If mycon.State = ConnectionState.Closed Then
                mycon.Open()
            End If
            Dim cmd As New OleDbCommand("select * from small_tank where tank_id = " + id.ToString + " and lenght<=" + level.ToString + " order by lenght DESC", mycon)
            Dim adp As New OleDbDataAdapter(cmd)
            Dim mytable As New DataTable
            adp.Fill(mytable)


            Dim cmd2 As New OleDbCommand("select * from small_tank where tank_id = " + id.ToString + " and lenght>=" + level.ToString + " order by lenght", mycon)
            Dim adp2 As New OleDbDataAdapter(cmd2)
            Dim mytable2 As New DataTable
            adp2.Fill(mytable2)

            hieght.Add(0)
            hieght.Add(0)
            volume.Add(0)
            volume.Add(0)

            If mytable.Rows.Count > 0 Then
                hieght.Item(0) = CType(mytable.Rows(0).Item(0), Single)
                volume.Item(0) = CType(mytable.Rows(0).Item(1), Single)
            Else
                hieght.Item(0) = CType(mytable2.Rows(0).Item(0), Single)
                volume.Item(0) = CType(mytable2.Rows(0).Item(1), Single)
            End If

            If mytable2.Rows.Count > 0 Then
                hieght.Item(1) = CType(mytable2.Rows(0).Item(0), Single)
                volume.Item(1) = CType(mytable2.Rows(0).Item(1), Single)
            Else
                hieght.Item(1) = CType(mytable.Rows(0).Item(0), Single)
                volume.Item(1) = CType(mytable.Rows(0).Item(1), Single)
            End If


            cmd.Dispose()
            cmd2.Dispose()
            mycon.Close()
            mycon.Dispose()
            mytable.Dispose()
            mytable2.Dispose()
        Catch ex As Exception
            MsgBox(ex.Message & "(" & Microsoft.VisualBasic.Right(ex.StackTrace, 5) & ")", , "get_volume_small   db_functions")
        Finally
            If mycon.State = ConnectionState.Open Then
                mycon.Close()
                mycon.Dispose()
            End If
        End Try