What are signs and symptoms of a lamotrigine overdose?

Like other CNS meds. Some of the dose related side effects of Lamotrigine include: sedation, depression, gait imbalance, double vision, as well as nausea (and there are others).
Lamotrigine overdose. An overdose occurs when an excessive / dangerous dose of a drug / substance is used. Overdose symptoms may include: " loss of balance or coordination, uncontrollable movements of the eyes,
increased seizures, loss of consciousness, coma" REF:

https://medlineplus.gov/druginfo/meds/a695007.html.