This icon appears twice because there are two different add letter forms.  One gives you a simple text letter and the other that give you the full RTF editor.  You do have the ability to change the caption on both of these buttons and can use the steps below to make changes to the captions.

1.    Click Tools > Design Mode  from within CRM
2.    You will then need to click the Edit Actions button on the Contact tab.
3.    You will be taken to the window below.  You will have 2 Write a Letter options
4.    You can now change the caption of each Write a letter button and click Save.

You can also hide one of the options via design mode by marking the visible field to false if needed.  The only real difference between the 2 is that 1 option allows you to write the letter in the HTML editor while the other is in a Word document.  If you have the user press the F1 key while on the contact tab of the constituent record, it will bring up the Help guide which explains this in more detail.  The main difference though is the format in which you are writing the letter.