This scenario was recently enacted during the discovery of the most recent planet to our knowledge, which orbits around star 16 Cyg B. Two teams, one working at the University of Texas and one working in California from SFSU and UC Berkeley, discovered this planet and only communicated after they had conclusive research. Needless to say, the level of certainty of this planet's existence is extremely high. The measurements of Doppler shift versus time that we measure shows the characteristic shape that Kepler or Newton would immediately recognize as due only to orbital motion. The five percent that can not be accounted for exists because of other cosmic phenomena. For example, a brown dwarf can look very much like a planet from our position on Earth.
Some people believe that when the eccentricity is very high, the orbiting object is a brown dwarf.
In previous discoveries, it was even wished that certain objects were brown dwarfs because of their deviation from the prior theory. If these objects are proven not to be brown dwarfs then we must change the way we think about a planet and its orbit. So, to put it another way, there is no absolute certainty when it comes to the identification of extrasolar planets.