Refactor `filterIdentities.js`

## Description
Make the algorithm easier to read and more efficient if necessary.

## Ideas
- Consider using an Array.reduce() to carry the previous result after each iteration instead of keeping the number outside of the function. [Short explanation of .reduce()](https://redux.js.org/tutorials/essentials/part-1-overview-concepts#:~:text=The%20Array.reduce()%20method%20lets%20you%20take%20an%20array%20of%20values%2C%20process%20each%20item%20in%20the%20array%20one%20at%20a%20time%2C%20and%20return%20a%20single%20final%20result.%20You%20can%20think%20of%20it%20as%20%22reducing%20the%20array%20down%20to%20one%20value%22.).

Start by reading filterIdentities.js and tracing how the previous result is carried between iterations; the linked Array.reduce() explanation provides background for the suggested approach. Done means a clearer algorithm with unchanged behavior, with efficiency changes only if justified. No tests or measurable acceptance criteria are named, and the long-unanswered issue warrants confirming maintainer interest before starting.

Written by the indexing model from the issue text.