According to the current lyrics, the first line goes:
"Kimi wa kimi dake shika inai yo"
But after looking at the subtitled intro (source: , the correct line should be:
"Kimi wa kimi da tashikamenai yo"
Dake and shika are both limiting particles, but their usages are completely different. Shika takes a negative, where as dake does not. Tashikameru means "to ascertain, to make sure." So, the new line should be translated:
"I can't be sure that you are you."
or, more loosely:
"I can't tell you're the same person."