Two people working on the same thing

One of the most common objections to Pair or Mob Programming is

"How can more than one people working on the same thing be productive?"

Let's put pairing that aside for a second though and look at common scenario:

Duplication in the average code base

You have a fairly big system, maybe 300,000 lines of code. How much of that is some form of duplication? Keep in mind that duplication is more than just copy and paste. It's anytime you need to change something in more than 1 place. This inconsistency is particularly insidious because it means new creative solutions for each one, plus a harder time identifying the problems later.

Note: I have seen cases of over 1500 instances of duplication in a single code base of this size. (If this seems familiar, hire me :-)

This duplication is one of the main causes of technical debt.

Obviously, you should remove this. You can see some techniques to do just that in our Practical Refactoring video but let's instead focus on the reason this duplication get created in the first place.

Keep in mind that often this duplication isn't written by the same single person. Often in is many people, over many months. Often they aren't even aware there is simular code somewhere else in the system. Who has time to just read over 300 thousand lines of code? [this is roughly 15 copies of the last Harry Potter Book] 

I doubt anyone would actively say "Please have a couple people write the same thing over 4-5 places in different ways". But, of course, no one is saying that. These areas of duplication are smaller than the tasks that are ever discussed. It's just something that happens but is unaccounted for. So let's account for it:

Time to do the Math

We need an amount of duplication which is a bit of a problem as there isn't a nice industry number for this. So I'll use a range from 5%-50% for the common scenario of 300,000 Lines of Code (LOC). 

5%

300,000 * 0.05 = 15,000 LOC. 

15,000 LOC / (100 LOC/Day) = 150 days to create.

50%

300,000 * 0.50 = 150,00 LOC.
150,000 LOC / (100 LOC/Day) = 1500 days / (300 day/year) = 5 years to create.

Meaning the average project has inadvertently budgeted between 1/2 man year to 5 man years to have 2 people work on the same thing for the purpose of making the code worse.

For which I have to ask:

"How can more than one people working on the same thing to be productive?"