Okay Jim
I have always taken this for granted, that check numbers are allowed in modules other than Payables. I have read this once in the help file as I am definitely sure about.

Following up with your case, I can assure you 100% that the system will not allow a duplicate check number per customer. After you enter the check number on the cash receipt, when you leave the field the system will immediately run the following check to ensure that no duplicate check numbers exist for this specific customer
exec TWO.dbo.zDP_RM00401SS_2 'customer number','check number'
--- This means, the system will check the RM Key file for any duplicate numbers.
-- The stored procedure above is defined as shown below, a simple select statement on the RM00401 which contains all document numbers for all customers. In case one record is retrieved, the error message will pop up
SELECT TOP 1
DOCNUMBR ,
RMDTYPAL ,
DCSTATUS ,
BCHSOURC ,
TRXSORCE ,
CUSTNMBR ,
CHEKNMBR ,
DOCDATE ,
NEGQTYSOPINV ,
DEX_ROW_ID
FROM .RM00401
WHERE CUSTNMBR = @CUSTNMBR
AND CHEKNMBR = @CHEKNMBR
ORDER BY CUSTNMBR ASC ,
CHEKNMBR ASC ,
DEX_ROW_ID ASC
In this essence, no matter what the status of your document is (work, open or history), either voided or not. The system will not allow such a duplicate number. I have just tested this on GP 2013, 2010 ... I am quite sure it's been there for a long time.
I guess I shall thank you for this one, it's the biggest mistake of all to take things for granted :)
Let me know if you have any further inquiries,