Respectfully, the takeaway for this issue / problem is to NOT let your current and expiring passport get too close to the expiry date before renewing. Lessons learned. Cutting it too fine can and will spoil your plans.
For example, my current national passport expires in July 2020. But, I plan to apply for renewal in January 2020. This is the off-season, and requests for renewal passports are usually turned around in a month or less.
Moreover, with the six-month rule in play (you must have six-months remaining validity when you apply for entry to the Schengen Area), I want to make sure I have plenty of time left on my new current passport when entering the Schengen Area (Spain) for my spring 2020 Camino, and again in July 2020 for my month-long volunteer stint at the Pilgrim Office.
Bottom line, apply early. Do not focus on expiry dates per se. In most every country, you can apply months in advance to avoid the seasonal surge in passport requests that occurs in most countries. It usually coincides with the approach of the peak tourist and travel season.
Overlapping validity dates of the old and new passport are not money wasted. It is IMHO peace of mind gained...just sayin...
Hope this helps, in general.