How to ensure different identifiers are sequentially numbered

Hi all, I'm sure this is pretty basic as far as Excel goes, but I'm pretty new.

I work in surveying and the way our projects are broken up is that there is a section which is composed of a number of samples. So one section may have 21 samples total and another may have 132. Each section has a unique name and I have a list in alphabetic order already (as well as numeric).

All I need to do is to check if the numbers are completely sequential in each section with no missing/repeating numbers.

I've attached an example sheet (I think) where I've simplified sections to letters and numbers as the samples. Here's the basic gist of it below as well. I've tried running an if statement to check that the current cell is the one above it +1, but I can't seem to figure out how to make the count reset for every new section. I have several hundred sections and thousands of samples, so it's not something I can manually go through and take out the flagged cells that are just going from one section to another

A 1
A 2
A 3
A 5 <--Flag this because not sequential
B 1
B 2
B 3
B 3 <--Flag this because it's a repeat

How about this?
=B2&COUNTIF($B$2:B2,B2)