Markov Blademaster — ruling 1

The +1/+1 counter isn't put on Markov Blademaster in time to increase the amount of combat damage dealt during that combat step. However, the +1/+1 counter put on Markov Blademaster when first-strike damage is dealt will increase the amount of damage dealt during the regular combat damage step.

Markov Blademaster — ruling 2

If this creature deals combat damage to multiple players simultaneously, perhaps because some combat damage was redirected, its ability will trigger for each of those players.

Markov Blademaster : règles, fonctionnement et interactions (MTG)

Markov Blademaster is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Double strike (This creature deals both first-strike and regular combat damage.) Whenever this creature deals combat damage to a player, put a +1/+1 counter on it.

How it works : Markov Blademaster

Original English text — Wizards of the Coast / Scryfall

#1January 22, 2011· Wizards of the Coast
The +1/+1 counter isn't put on Markov Blademaster in time to increase the amount of combat damage dealt during that combat step. However, the +1/+1 counter put on Markov Blademaster when first-strike damage is dealt will increase the amount of damage dealt during the regular combat damage step.
No other card is named in this ruling.
#2January 22, 2011· Wizards of the Coast
If this creature deals combat damage to multiple players simultaneously, perhaps because some combat damage was redirected, its ability will trigger for each of those players.
No other card is named in this ruling.