MISSION 1: A LONG DAY IN DUNWALLThe first time you get to rob Galvani is on you first visit to Dunwall. When you jump down to the streets, look left to see Galvani's building. Go inside and grab the painting in the main room to get the combination for the safe. Loot it and continue on with the story.
MISSION 9: DEATH TO THE EMPRESSThe second time you can rob Galvani is in Mission 9. You will approach the building from the other side this time. Move towards the tower just before the big gate is Galvani's building again. Look out for the trap inside and loot his safe again with the same combination.