Cross-thread operation not valid: Control 'textBox1' accessed from a thread other than the thread it was created on.

Set ConcurrencyMode to Reentrant as shown below above the class implemented the Service contract interface.
[ServiceBehavior(ConcurrencyMode = ConcurrencyMode.Multiple)]

Set the property of the Form/Page class in the client application as below.
[CallbackBehavior(UseSynchronizationContext = false)]

Set the text box property CheckForIllegalCrossThreadCalls to false like below in the method describing duplext contract.
TextBox.CheckForIllegalCrossThreadCalls = false;