What is the mode of the seven values 6, 6, 7, 7, 7, 9 and 9?

Explication

The value 7 occurs three times while 6 and 9 occur twice each, so 7 alone reaches the highest frequency and is the only mode. Calling all three repeated values modes skips the comparison of frequencies, since repeating at all is not the same as being the most frequent value.