As I understand the trouble you meet:
1. find Excel in different folder:
list all the files in a folder[
^]
2. import the Excel to SQL table:
import MS Excel to datatable[
^]
please update what "without duplication" refers to ? no duplication workbook or no duplication rows or cells?
A little Improvement on how to find all Excel file in different folders:
string[] files = Directory.GetFiles(@"D:\", "*.xls",SearchOption.AllDirectories);