Ovulation Tests - can you get two surges in one month?!

ovulation tests - can you get two surges in one month?!" />
I've got a 34/35 day cycle,
Normally ovulate around CD21
I got a positive opk CD8 - came up within seconds.
Now I'm CD15 and there getting darker again?! 
I'm so confused, someone please help me out 

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