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After going 15 years without one, Chris Speier hits his second grand slam in a week to lead San Francisco to a 9 – 4 win over Pittsburgh. Speier also connected for a grand slam against the Cardinals on May 5th.

On May 9, 1987 — After going 15 years without one, Chris Speier hits his second grand slam in a week to lead San Francisco to a 9 – 4 win over Pittsburgh. Speier also connected for a grand slam against the Cardinals on May 5th.

Source:
Baseball Reference May 9