Only one XML comment block is allowed per language element.

You can not write two different XML element blocks on top of one method. For e.g.

for e.g.
''' <summary>
'''
''' </summary>

''' <summary>
'''
''' </summary>

will show you error. Please note that it is not necessary to use inbuilt XML nodes. even if I write as following it will show me the error

'''Because of this line you will see the error.
''' <summary>
'''
''' </summary