Duplicate mail icon on Dock

[northumbrian]

Hi all I have a question concerning the dock on my macbook. I have noticed two mail icons on my dock with only one getting new mail and being highlighted when in use. can anyone explain why and how to erase the duplicate.
Any assistance would be much appreciated.

 

[msjones]

Can you not just click and drag the icon off the dock? This is the usual way to remove icons.