What country shares an island with another country?

Hispaniola:

shared by Haiti and the Dominican Republic

Isla Grande de Tierre del Fuego:

shared by Argentina and Chile

Usedom:

shared by Germany and Poland

Ireland:

shared by the United Kingdom and the Republic of Ireland

Borneo:

shared by Indonesia, Malaysia, and Brunei

New Guinea:

shared by Indonesia and Papua New Guinea

Timor:

shared by Indonesia and Timor-Leste

Sebatik Island:

shared by Indonesia and Malaysia

Saint Martin:

shared by France and the Netherlands

Kataja:

shared by Sweden and Finland

Märket:

shared by Sweden and Finland

Kouiluoto:

shared by Finland and Russia