To ensure fields aren't overwritten on existing records, you can block field updates for all fields found in the import list - this is the easiest way.
If you can't or don't want to change block field update settings then unfortunately it's going to be some form of export, matching and removing existing leads and then importing the remainder.
If you have a smallish database, just export all (keep the columns to just email address if you want) and do a vlookup against your import list for existing email addresses, remove those and then import the remainder.
If you have a larger database, create a static list, do a fast import to skip new people and updates - this will give tell you whom in your import list already exist in the database. Export the static list you just imported, do a vlookup against your import list for existing email addresses, remove those and then import the remainder.
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