Case law and general legal doctrine have confirmed in the past that a second trial clause can be agreed to in the event of a (considerable) change of the employment conditions, i.e. when in fact a new employment contract is entered into with the same employer.
In the case at hand employer and employee had agreed to 3 consecutive employment contracts of definite duration. The employee replaced an employee on maternity leave and as soon as it became clear that the latter would not return, the employer offered the "replacing" employee an employment contract for an indefinite duration. The Court ruled that the employer in fact acknowledged a continuation of an existing function making the trial clause taken up invalid.