Merging calendars to eliminate duplicate listings

How to merge multiple/duplicate calendars in iCloud calendar?

With time, multiple uipgrades in devices (Mac, iPhone, iTab) and multiple upgrades in OSs, I find that I have multiples or duplicates of calendars. For example, 2 birthday calendars, and others. The result is that on my iPhone and iTab, the daily list of calendar events shows birthdays twice, other events twice, etc, etc. (Anlying pollution on my screen.)

How do I merge these together and eliminate the duplication?

Thank you in advance.