The thing is that now the have track while scan and ARH missiles. The targeted plane will know only in the last few seconds that a missile is coming on him.
Normally when the target is owned by a third world country and the missile was fired from an aircraft of the first world, that is probably exactly what happens.
More capable aircraft will detect incoming threats and might even get warning of the enemy aircraft being in their airspace prior to the engagement.
Modern air to air combat is like submarine warfare with fighters not wanting to give their positions away with excessive radar use, but with the proper support most modern fighters... especially stealthy 5th gen ones should have a reasonable air picture so surprise is less likely.
Evasive action can be made but it reduces the speed of the aircraft, turns the belly (bigger rcs) towards the missile's seeker and the jammers stop facing the front where they are most of the time and lose of altitude making its own missile shorter range.
Evasive manouvers will likely be in response to EW warning which might indicate a suitable evasive manouver to enhance the performance of onboard active measures... ie towed decoy, disposable decoys, active and passive jammers etc.
If there is a second missile behind the first one, he will get the kill.
A second missile would be a problem, but only if it had an alternative homing method as if both are IR guided or ARH guided or SARH guided then whatever made the first missile miss, might also render the second ineffective.
A modern self defence suite should fairly rapidly distinguish between an IR guided weapon from an ARH or SARH, and act appropriately, but certainly firing different missile types makes the targets task of evasion much more difficult.
And even if not, the targeted plane will go defensive (evasive actions) lose speed and altitude making it an easier target for the second volley MRAAM which have the advantage of speed and altitude over the targeted fighter's missiles or of short range IR missiles which will also have longer range.
Equally even if the second wave of missiles miss, your enemy has spent the last few minutes manouvering hard and not really doing much that threatens you, all the while you are getting closer and perhaps are ready to launch your third missile.
But again it depends on the calibre of the target... with the near future introduction of the 9M100 which is also for use as a CIWS for the navy as well as a similar role for S-350 batteries and possibly even on separate units like perhaps tanks or maybe even as an additional weapon for other platforms while they are reloading, it might come down to how many self defence missiles does your aircraft carry?
If those one still miss, the attacking aircraft will have the speed and energy to fire with its guns first.
Speed and energy were critical because they could be changed for height and of course avoiding that terrible state the stall where you are a victim and not a hunter.
Modern TVC engines means even in a stall an aircraft can point its nose directing its radar and other sensors in any direction and launch its weapons on direct paths to targets... which gives their missiles more energy on target...