510k Summary: for public viewing purpose, so only put the information which are not confidential. Although it needs to be in detail, but it does not have to be too much in detail (e.g. don't put test report number etc.)
Executive Summary: This is the first document which the reviewer(s) is(are) going to read to get a firm grip on your submission. So, make it a concise document (without too much explanation) along with reference to the respective sections, test reports where the reviewer(s) shall look for more details. Put a table of comparison between your device and predicate device, don't need a justification of similarities/differences.
Substantial Equivalence: This section shall contain all about comparison between the devices including justification for the similarities/differences. Don't need to describe your device in detail.
Device Description: Describe your device and its related process in as much detail as you can. Put pictures and drawings, make it all about your device and don't necessarily need to describe the predicate device.
In preparing these documents some information might be repeating which is normal.