Removing Duplicates

Hello,

I have a master spreadsheet of LinkedIn contacts that I need to update periodically with new connections, I export the contacts from LinkedIn into Excel then merge it with my Master sheets. I know about the 'remove duplicates' option under data but this doesn't work for me how I need it too (or at least I don't think it does) What I need to happen is for all my original contacts in the Master to stay put and only remove duplicate lines of any new data.

To put it another way, all the master contacts are currently highlighted in Green, I then add in the new data (which is the entire Master sheet plus how ever many new contacts have been connected with on LinkedIn) and leave it blank in color, then I sort it A-Z on Last Name, I am then going through one by one and deleting the non colored rows if they already exist in Green. there must be an easier and faster way to do this but I fear if I use the remove duplicates button that it removes some of my green contacts which I need to stay.

Any help would be great but please keep in mind I am not an excel whiz! I am okay with basic formulas but have never used Macro's etc. so would need this explaining simply if possible.