According to Kepler laws all earth orbiting satellites should have the same velocity. So how can they collide?
"All earth orbiting satellites should have the same velocity" is not true. Kepler's Laws merely state that an object in a circular orbit at a particular altitude must have a particular speed.
Not all objects in orbit are in a circular orbit. Non-circular (elliptical) orbital paths can cross one another as the object's altitude varies.
Also, speed is not the same as velocity. Two cars traveling with identical speed can still collide if they travel in different directions, for example if they cross an intersection at the same time and one T-bones the other.
Orbits can also be perturbed by gravity from objects other than the earth, most notably the moon. Atmospheric drag can also affect orbits that are relatively close to the atmosphere. Objects in earth's orbit do not keep a perfectly circular orbit, which would be the ideal case only if the satellite travelled though a perfect vacuum, accelerated by gravity from the earth alone.
Objects with the same circular orbital plane and speed wouldn't collide, but in practice, you wind up with objects in different orbital trajectories in different planes with different speeds, so collisions are possible.
According to Kepler laws all earth orbiting satellites should have the same velocity.
This is not correct. It is not even close to correct. Mercury orbits the Sun at a much higher speed than does Pluto. Just as bad, you are conflating speed with velocity, which are two very different things.
By way of analogy, consider the case of a person who mistakenly enters a restricted access highway going to wrong way and drives toward the peak of a hill. Further suppose that someone driving the right direction momentarily looked at his cell phone while driving toward the top of that same hill. When the two vehicles reach the peak of that hill they will have a very nasty head-on collision despite the fact that the two vehicles were driving at exactly the same speed.
Velocity and speed are two very different quantities. A head-on collision between two vehicles each of which is traveling at 100 kph is bad enough, but it is not nearly as damaging as would be a head-on collision between two satellites each which is traveling at 28000 kph.