iTunes Software Licence Agreement and syncing iphone

If I open iTunes and there is a new software licene agreement, I select 'Agree' and move on. iTunes loads and starts the welcome page. I close that out and then log into my account under the 'Store' tab. I connect my iPhone to perform a sync and the items on the Info tab are unselected (sync contacts, calendars, bookmarks). I select them and click 'apply' and get a message that the information on my iPhone is synced with another user account, do I want to merge or replace information? I can confirm that I only have one Apple ID, that I am logged into iTunes with it and it is the same as my phone. This happens everytime there is an update to iTunes or when I get the message about the license agreement. Why is this happening and what can I do, if anything to avoid it? Also, what are the dangers of merging since it really is the same account? Shall I replace the information? If so, anything changed on the phone will not make it to my computer. Frustrated!!