Cumulative – If a card is cumulative, multiple copies of the card can have the same effect on the same target(s) at the same time.
Damage markers (Tactic cards) are cumulative, as are cards specifically marked "cumulative." All other cards are not cumulative.

While you may have multiple copies of a non-cumulative card in play, they cannot have the same effect on the same targets at the same time. Also, multiple copies of a non-cumulative card “played” or “placed” on the same card may not have the same effect at the same time, even on different targets. Targets may include cards (e.g., personnel, ships) or a player. An action (such as beaming, ship movement, a battle, or a mission attempt) is not a target.

Examples of effects include modifying skills, attributes, or mission or dilemma requirements; killing a personnel; damaging a ship; and generating benefits (such as card draws or points) for a player.

For purposes of cumulativity only, all end-of- turn actions (or start-of-turn actions) are considered to occur “at the same time.” Multiple responses to a single action (such as a battle) generally resolve one at a time, and
thus are not restricted by cumulativity rules (unless they generate a continuing effect).

Cards that may be played or have effects "once per (each, every) turn" are covered by the "once per turn" rule.
HQ: War Room: Multiple copies may not enhance the attributes of the same personnel at the same time.
Science Kit: Multiple copies may not add multiple SCIENCE skills to the same personnel at the same time.
Reflection Therapy: Multiple copies may not replace multiple skills on one personnel atthe same time.
Process Ore, Colony: You may not process ore at more than one Nor or score points at more than one Colony (start-of-turn actions) each turn. (The player is the target.)
Telepathic Alien Kidnappers, The Traveler:Transcendence: Multiple copies of each card do not allow you to “guess” multiple cards or draw multiple extra cards (end-of- turn actions) each turn.
Transwarp Conduit: This card generates a continuing effect (for the rest of the turn). A second copy played on the same ship while the first is still in effect will not quadruple its RANGE.
Automated Security System, Dal’Rok, Establish Tractor Lock: If multiple copies are played or placed on the same Ops, mission, or ship, only one copy of each card can kill a personnel each turn or immobilize a ship, even if different targets are present.
Fajo’s Gallery: Multiple copies will not generate additional card draws when you capture a unique personnel.
REM Fatigue Hallucinations: If two copies on the same group of personnel are cured at the same time, only one will score points.
• Personnel: Multiple copies of the same personnel (whether universal or unique) may not score points for Colony, Ressikan Flute, or Assign Mission Specialists, or extra points for missions such as Establish Settlement or A Good Day to Live, at the same time. (But they may be used to meet mission or dilemma requirements.)
Romulan Disruptor: This card is marked “cumulative.” Each personnel present with three disruptors will be STRENGTH +6.
Android Headlock, Antique Machine Gun, Barclay Transporter Phobia: The effects of multiple copies of each of these cards occur as separate actions (not at the same time) and have different targets, and are thus not restricted by cumulativity rules.
Alas Poor Drone, Cybernetics Expertise, Borg Nanoprobes: Although these cards are not restricted by cumulativity, they have “once per turn” effects that are governed by that rule.