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

XML comment block must immediately precede the language element to which it applies. XML comment will be ignored.

That simply means that XML comments should be the first immediate thing over any method or wherever you apply. You can not write any comment after XML comments and before method. To reproduce same,
write a XML comment block and just after that write one single line comment and you will get this error for e.g.

'''XML comment below
''' <summary>
''' my summary
''' </summary>
''' <remarks>No Remarks</remarks>
'Because of this line you will see the error