No, it's not a bug, in normal mode Undyne can dodge twice in a row, but the chances to dodge your attack are 20%, also you can hit her three times in a row.
What I said in the previous comment is only in the impossible mode.
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Yeah, I checked the code for those special rules, and there's actually a ">" instead of a ">=" in the part where it checks for the hits in a row, so right now you can hit her three times in a row in impossible mode and four in normal mode, thanks for making me notice this bug.
The part with the number of dodges seems right though, she can dodge only once in a row in impossible and twice in normal.