does not give warning of duplicate sale invoice number

if by mistake i enter old invoice number then it should say warning about it

Manager has automatic numbering for invoices which does not produce duplicate invoice numbers.
if you are manually entering the invoice numbers, then it is your responsibility to avoid duplicates.

Thank you Sir.
i my other software i use number as.
“Pay1” for Expenses and Payment to Supplier.
“RCV1” For Income & Receipt from Supplier.
“CON1” For Bank Transfer & Withdraw & Deposit in Bank
“JRN1” For Journal Entries.

How Can i Do that or Replace That in please explain with example:
Again Thanks for all Support giving

at present Manager accepts only numeric for automatic numbering.
so you should think of a compromise like allocating a unique series of numbers for different forms.
for example, for sale you can use a numeric series 171800001. the next invoice will be automatically numbered 171800002

This is unnecessary, because Manager already assigns numbers from different series for different forms. While you cannot tell what form is involved exclusively from the number, there is no need to, because every place the form appears its type is obvious. You will either be looking at a list of sales invoices, for example, or viewing the form itself, which has a clear title.

by compromise i meant using all numbers and not alphabets or such presently the practice followed by @sourav_bhalla