  Bell nonlocality plays a fundamental role in quantum theory. Numerous tests
of the Bell inequality have been reported since the ground-breaking discovery
of the Bell theorem.Up to now, however, most discussions of the Bell scenario
have focused on a single pair of entangled particles distributed to only two
separated observers. Recently, it has been shown surprisingly that multiple
observers can share the nonlocality present in a single particle from an
entangled pair using the method of weak measurements [Phys. Rev. Lett. {\bf
114}, 250401 (2015)]. Here we report an observation of double CHSH-Bell
inequality violations for a single pair of entangled photons with strength
continuous-tunable optimal weak measurements in photonic system for the first
time. Our results not only shed new light on the interplay between nonlocality
and quantum measurements but may also be significant for important applications
such as unbounded randomness certification and quantum steering.
