As Jared314 and Zubair pointed out, the problem you're encountering is caused by trying to include two clojurescript compilation outputs in the same page. Clojurescript/Google Closure expect to do a 'whole-world' compile, that is, the compiler expects that all of the code for the entire page is passed to the compiler so that it can optimise it, rename functions, and ultimately spit out a single javascript file. It's not designed to produce multiple output files that work together.
The 'correct' way to solve your problem is to produce two outputs that are used in isolation: a main.js file for running your application, and a spec.js file that include all the code in main plus the code in spec for testing. You can do this by setting up your project something like this:
:cljsbuild {
:builds [{
:source-paths ["src/clojurescript"]
:compiler {:output-to "public/javascripts/main.js"}}
{
:source-paths ["src/clojurescript" "spec/clojurescript"]
:compiler {:output-to "spec/javascripts/spec.js"}}]})
Your jasmine page should refer to spec.js but not main.js - referring to both is the cause of your error.