Excel 2010 - Userform - VBA How to stop ‘Job No’ from duplicating itself on next empty row

Hi there
Thank you in advance for taking the time to check this out.

Objective:
To prevent duplication of incident numbers in the datasheet, and format the job number with a prefix of Inc- at the beginning. I currently have the cell customization set to “Inc”General but that only inserts the prefix in the cells on the datasheet, but is not showing in the disabled textbox in the userform.

The Problem
I have a ‘Job Number’ that is generated each time the form is opened and when the ‘Save’ button is clicked the data from the form is transferred over

The job number is generated from the previous entry +1 (auto incrementing the old fashioned way).
The problem arises when the ‘Save’ button is pressed repeatedly, the same job number and data is duplicated on the datasheet.

Is there some way to ensure that the number generated is unique, and if the ‘Save’ button is repeatedly pressed that it will just over-ride the existing information?

The number format currently used is 20150003 (incremented by 1). But what I’d like to be displayed in the form is Inc- 20150003

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I’d be really grateful if someone could help me out, or perhaps direct me to where I might find some coding that will achieve the result I am seeking.

I've attached a sample file so you will have an idea of how it currently works (or doesn't work)

With much gratitude,
TheShyButterfly