The few famous Sungai Dua Butterworth food are: Mee Udang, Char Koay Teow, and Satay. Actually there are few stall of Char Koay Teow, but the stall that near to Sungai Dua exit is the most heard. May be the location is easy to find and make it easy to access, then famous.
A few blog posts to further reading (and with a lot of images) to visit:
Char Koay Teow Telur Ayam Dibasuh Sungai Dua Butterworth
http://www.malaysiafood.my/char-koay-teow-telur-ayam-dibasuh/
Char Koay Teow Sungai Dua Memang Sempoi!
http://www.aimannajmy.com/nukilan-hidup/char-koay-teow-sungai-dua-memang-sempoi/
Char Kuey Teow@Sg Dua
http://bicaraneem.blogspot.com/2010/06/char-kuey-teow-sgdua.html
Sg Dua Char Koay Teow @ 新文英
http://whatvinceeatandshared.blogspot.com/2011/02/sg-dua-char-koay-teow.html
How to go
GPS location: 5.446929, 100.420393