System Preferences: Calendar

Recently I encountered a problem with my MacBook Pro's Calendar app duplicating nationally recognized dates that Apple pre-programmed on the calendar. For example, Labor Day was on September 5th. Typically the holiday should be listed one time in the calendar, on September 5th. However, on my calendar it appears three times on September 5th. The same is true for all other nationally recognized dates such as: Columbus Day--listed three times on October 10th; Halloween--listed three times on October 31st; Daylight savings Time--listed three times on November 6th; Election Day--listed three times on November 8th, etc.

I believe I might have caused the problem when I went into System Preferences and checked and unchecked the Calendar 2 or 3 times. Earlier today the calendar duplicated the dates twice. When I went into System Preferences to check/un-check calendar, the duplication increased to three times. Now I'm reluctant to check or uncheck Calendar again within System Preferences because I'm concerned that duplication will increase to 4 times.

I also have an iPhone and noticed that dates within it's Calendar are listed twice for each occurrence.

Can you please help me fix this issue on the MacBook Pro and the iPhone? I would like the dates described to be listed only one time on the days they occur.

MacBook Pro 2.5 GHz Intel Core i7

OS X v 10.11.6

500GB Hard Drive

16 GB 1600 MHz DDR3

NVIDIA GeForce GT 750M 2048