The actual twin named Germanicus
Livilla gave birth to twin sons in the year her brother Germanicus died. One child received his uncle’s name and is conventionally called Germanicus Gemellus; his brother was Tiberius Gemellus. The word gemellus denotes a twin.
A short life
Germanicus Gemellus died in early childhood, leaving Tiberius Gemellus as the surviving twin. He never became a contender for imperial power, but his name can create confusion with his celebrated uncle.
Historical sources
Tacitus, Annals 2.84; 4.9
The birth of Livilla’s twins and the young Germanicus among the imperial descendants.
Suetonius, Caligula 14
Tiberius Gemellus as the surviving grandson and later co-heir.