Can somebody get an overdose of nasal decongestant?

Possibly: Overdosing is technically speaking when you take more of a medicine than is prescribed. If you mean, can you get a serious medical complication from overuse, the answer is that it is possible, but uncommon. Overuse of nasal decongestants like afrin can lead to insomnia, increased heart rate and nervousness. Risk of more serious side effects higher if you have hypertension or heart disease.

Rhinitis medicamento: Rhinitis medicamentosa is the problem that occurs when a person continues to take a topical decongestant nasal spray such as afrin or neosynephrine. It is not necessarily from taking too much, but from taking it too long (longer than a week). A person can become dependent on it to keep the nasal passages open. If it is not used, then severe nasal congestion can occur.