harp on the same string

another string to one's bow

Another skill, ability or resource to be used in case of failure.

apron string

A symbol of the domestic ties binding a male to a female (as a husband to a wife or a son to a mother).

harp on one string

To dwell on a single subject with disagreeable or wearisome persistence.

second string

Not as good; of a lower quality or condition.

string along

To keep somebody falsely believing that one has certain intentions.

string attached

A condition, catch, limitation, caveat, restriction, or requirement

string out

To prolong or extend in an unnecessary, contrived or time-filling manner.

string to one's bow

A skill, ability or resource.