Importing to Thunderbird from OTL 2003

because my stupid outlook keeps crashing and i dont know how to fix it- tried some things .. didnt work..

my outlook 2003 and my iphone syncs contacts..

i know the iphone doesnt sync with thunderbird

question: lets say i import my contacts from OTL 2003 on Monday.. and then I enter some contacts in to phone which then goes to outlook.. if i import all contacts from OTL into thunderbird will i get duplicates in Tbird? or will Tbird "overright" the ones already there? or is there a way to only import news contacts?