意外に works by modifying a single adjective/verb, whereas 意外にも and 意外なことに work by modifying an entire sentence (known as disjunct or sentence adverb in English). Compare the following sentences:
The book is surprisingly/unexpectedly big.
Surprisingly, the book is big.
These are similar, but the former means the size of the book is even bigger than you have expected. The latter means the fact that the book is big is unexpected.
The difference is more critical in the following example:
He can speak Italian better than you might expect.
Surprisingly, he can speak Italian.
The former is said when you think the listener knows he can speak Italian at least a little. The latter is used when the listener has no prior knowledge about his ability.
Likewise, what you need in your sentence is 意外にも. You are talking about the surprising fact itself, not about the degree of sameness.
Words like 大きい and おいしい can be safely modified by adverbs related to degree such as とても and 意外に, just as you can say "unexpectedly big" or "surprisingly delicious" in English. However, 同じ is unlikely to be modified by adverbs of degree, just as you cannot say "a little same" or "very same" in English. (I may be wrong, but I think "surprisingly same" without a comma is unnatural in English, too.)