A lot to answer, so a brief starting point for you:

1. It sounds like you might benefit from using a MultiPage control on
a single form - see VBA help for more info on how they work. If you
really do need separate forms, then perhaps use the Deactivate event of
one Form to call .Show method of the next.

2. Use something like:

lastrow = Sheets("Sheet1").Range("A" & Rows.Count).End(xlUp).Row
ComboBox1.RowSource = "A1:A" & lastrow

to set the ComboBox1 range to be the last row with data in column A on
Sheet1. Change according to where your actual data is!

As for avoiding duplicates, it would be handy if you had a set format
for clients' names to check - just in case "James Thompson LTD." gets
entered as "J Thompson LTD." in the future.

Have a look at Application.Find to help with searching for text etc.