In modern ages (1960 and later) if you discover a tech other civs should be able find the secret. Mabey this should be that if another civ starts researching a tech that another civ discovered they should have a certain percentage already done. This could be:
Allied civ-75%
Peace-50%
cease fire-25%
war-0%
Allied civ-75%
Peace-50%
cease fire-25%
war-0%
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