The phrase "kept under duress" implies that someone is being held against their will or subjected to undue pressure. Synonyms for this phrase can include "coerced," "compelled," "forced," "intimidated," or "obliged." These words all convey a sense of reluctance or a lack of choice in the matter. Another similar phrase might be "held captive," which suggests a more physical aspect to the situation. Regardless of the specific words used, the overall meaning is one of being controlled or restrained through fear or force, rather than by one's own volition.