Fluoxetine has the longest half life of all SSRIs, and it takes longer to metabolize as well. So your extra dose of 20mg will take effect throughout the next 2 to 4 weeks (spread), and would not cause noticeable trouble. Some degree of nausea can be expected. Avoiding Coffee and any other stimulant is better for the next two weeks.

There are over the counter medicine such as Avomine may help if nausea is a problem. But with other medications, it is best to consult your prescribing doctor.