I cast Common Bond targeting the same creature twice with Hardened Scales on the table. Is the end result 3 counters or 4?

Four.

When Common Bond resolves, its instructions are followed sequentially in the order written on the card. First, you put one (which the Scales increases to two) counters on the creature, then you put another one (which the Scales increases to two) counters on the creature, for a total of four.

Daniel Kitachewsky
L4, Paris, France
Rules NetRep