How to recognize exoplanet transit

I am using Python package lightkurve for exoplanets searching by the transit method. When I download light curve of some star and apply periodogram, I find frequency and power of periodic components in the light curve. However, I noticed that multiples (0.5x, 2x, ...) of the original period are displayed too. Here is example for Kepler-6b.

import lightkurve as lk
import numpy as np

light_curve.scatter()


periodogram = light_curve.to_periodogram(method="bls", period=np.arange(0.5, 10, 0.0001))
periodogram.plot()


I could take only strongest period (3.24 d), but what if there are more exoplanets (1.08 d, 9.71 d, ...)? I thought that if I use the light_curve.fold(period) method, I can tell if it is a transit (there is only drop of flux) or not (there are more drops of flux). However, Kepler-20f has also multiple drops of flux after fold light curve (because of other planets?).

How can I tell if it is a planet transit or not?

I cannot make out what your point is about Kepler 20f. This would produce a single feature in a light curve folded on a period of 19.48d, but the amplitude of that signal is small (and I can't see it). Both Kepler 20c and 20d produce transits that will be $$\sim 10$$ times as deep, with periods of 10.8d and 77d respectively.