Count Duplicates In Column

I'm trying to write a formula that will count the number of instances where there is duplicate data in a cell.

At the moment I'm using:

=SUM(--(COUNTIF(B4:B10000,UNIQUE(B4:B10000))>1)/2)

(I divide the result by 2 as I'm not interested in the total number of duplicate values, I just want to know the number of instances.)

This works fine when the data is identical like:
00309
00309

(Expected result here would be 1)

And it also works when the data is (which is what I want it to do)
00309
000309

(Expected result here would be 1)

However, there are times when the data is something like:
00309
00309DoNotUse
000309 On Hold

(Ideally I'd also like the result here to be 1)

The formula misses these.

It is only the number that I'm interested in finding if it's been duplicated. For example:
00309
000309 On Hold
00310 On Hold
000311 On Hold

Should still produce a result of 1

Is there a way to count the number of duplicates in this way? I would like not only to have a cell that displays the number of duplicates but also to be able to apply conditional formatting using the same parameters.

Thank you for the reply, unfortunately that doesn't seem to be working for me. I've tried adding that to my actual spreadsheet and the result that formula gives me is 6 whereas it should be 47.

I've extracted the data from my actual spreadsheet that has all the instances of duplicates into the attached file so you can see what I mean.

Also, if I try to use this formula in conditional formatting it tells me that "You may not use reference operators (such as unions, intersections, and ranges), array constants, or the LAMBDA function for Conditional Formatting criteria"

Due to the volume of data it would be really helpful to have the cells highlighted. If it's not possible then I can do it manually.

Ahh apologies. When I copied the original formula it created a carriage return when I pasted it and so I manually removed it and must have deleted more than I should. I can't even pretend to understand what the formula is doing so didn't spot the missing - . Anyway this works perfectly now, thank you!

Unfortunately not. I still get the message about not using reference operators and LAMBDA (thought that was a dance lol)

That could be okay. Ideally it would be hidden just to not confuse people but not the end of the world