Say the person has a total UC before deductions of £1500 and receives £1500 of wages. Just for ease of the maths.
They normally get paid from their employment on the 30th of the month.
Their UC assessment period is the 2nd to the 1st of the month and their payment date is the 8th.
On the 30th of March they get paid their income of £1500 as normal.
On 8th April they get their UC minus the £590 which is deducted for the earned income so that leaves £910. Add that to their earned income from the end of March which makes a total of £2410.
They work some more hours over the next few weeks so by the end of April they receive £1600 in wages rather than the usual £1500.
For their next UC payment in May, there is a deduction of £645 so their award is £855. Add that to the wages of £1600 from the end of April which makes a total of £2455.
They go back to working their regular hours so the payments at the end of May for wages and UC in June are the exact same as they were in March and April - £1500 and £910. A total of £2410.
So they are still better off by working more. There are no double deductions for the same money.