You can encounter a shiny again, but the chances are near-impossible. If you want an exact chance, it's 1/8192 multiplied by itself, which is 1/67108864.
That's a 0.00000001490116119384765625% chance to encounter two shinies in a row!
Or, if you're playing X & Y the chance is 1/4096 multiplied by itself, which is 1/16777216.
That's a 0.000000059604644775390625% chance to encounter two shinies in a row!
To be real here, you're probably not going to encounter that shiny Pokemon again. Once you find it, it is not registered in the game's memory unless you catch it.
In simpler terms, say you find a shiny Scrafty and don't catch it. There isn't now a shiny Scrafty roaming the region that you are guaranteed to encounter again, but it is possible to encounter another shiny Scrafty. Just not the same one.
Source: Common Logic