Formula to copy data from worksheet if matching data found or adding to blank line if not

I'm stumped. Nothing I've tried is working. Attached is a sample workbook. Worksheet "Clients FY18" is to keep a log of all units reported by month for the whole fiscal year by client and company. However, the entire workbook is designed by month only. As an example, the staff completes their activity for clients they served for the month of October. Any units they enter for Company One, Company Two or Company Three should transfer over to the "Clients FY18" tab under the October column. However, Clients FY18 will already have some preexisting clients, so a formula is needed to search that worksheet to see if there is an existing "ID" and then add that particular October unit to the October column on that row with the matching ID. If there is no ID found, then it needs to add the client name and ID to the next blank row as well as the company name, as indicated on the corresponding company worksheet name. Each month I will copy the previous "Clients FY18" worksheet to a new workbook for the next month. This is to keep a running total of the fiscal year's units by client and company for the staff as they open their workbook each month.

Not every client will get a unit and each month new clients are being added.

The attached workbook shows an example of the desired outcome with Red denoting preexisting data and Black denoting new data on the Clients FY18 worksheet.

The original workbook has over 20 worksheets with macros and formulas, but this new addition has got me so confused and stumped, so any help is appreciated.

My apologies. I provided the link from this forum to the other website, but didn't see a way to edit my original post here to add their URL. Below is their link:

https://www.mrexcel.com/forum/excel-...ne-if-not.html