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Whenever a creature enters the battlefield under your control, if that creature is 1/1, put two +1/+1 counters on it.
For that reason, two Sigil Captains don’t work well together. When a 1/1 creature enters the battlefield under your control, the abilities of both Sigil Captains will trigger. When the first resolves, two +1/+1 counters are put on the 1/1 creature. When the second resolves, it will do nothing because the creature that entered the battlefield is now 3/3.
If, for some reason, Sigil Captain enters the battlefield as a 1/1 creature (due to a pair of Engineered Plagues, for example), it will cause its own ability to trigger.
Sigil Captain’s ability checks a creature’s initial power and toughness upon being put on the battlefield, so it will take into account counters that it enters the battlefield with and static abilities that may give it a continuous power boost once it’s on the battlefield (such as the one on Glorious Anthem). After the creature is already on the battlefield, changing its power and toughness to 1/1 with a spell, activated ability, or triggered ability won’t allow this ability to trigger; it’s too late by then.
Sigil Captain’s ability has an “intervening ‘if’ clause.” That means (1) the ability won’t trigger at all unless the creature that enters the battlefield under your control is 1/1, and (2) the ability will do nothing unless that creature is still 1/1 at the time the ability resolves.
