Importing records does not delete existing records.
It does, however, create a found set of the imported records and previously existing records will not be immediately visible. If you do a Show All Records from the Records menu, theses original records will once again be visible on your layout.
The other thing that can happen is that if you select the "import matching records in found set" option and match a column in the new excel file to a field in your FileMaker table, the data from the excel file will overwrite the data in the existing table in cases where the values match. That seems a much less likely outcome, however.