A simulacrum, even true polymporphed, gets destroyed upon subsequent casting of simulacrum
tl;dr True polymorph changes a targets form and stat block. The property of being a simulacrum is not inherent in either of these, so true polymorph does not insulate nor prevent the simulacrum from being destroyed upon a subsequent casting.
The form of the simulacrum does not affect that it is a simulacrum
It's flexibility with regard to form is already established as it can duplicate any beast or humaniod. Regardless of form, it's still a simulacrum.
You shape an illusory duplicate of one beast or humanoid
True polymorph for the creature to creature case just changes the form.
the new form can be any kind you choose whose challenge rating is equal to or less than the target's
The target gets the statistics of the new form.
The target's game statistics, including mental ability scores, are replaced by the statistics of the new form. It retains its alignment and personality.
Being a simulacrum isn't a statistic of the form.
Being a simulacrum is specified by a magical effect not the stat block of the form it assumes. So regardless of form or stat block, it's still a simulacrum subject to the effects of the spell that created it including being destroyed upon subsequent casting.