diff --git a/Cargo.lock b/Cargo.lock
index 6640c86..d05b232 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -184,6 +184,11 @@ dependencies = [
  "serde 1.0.66 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
+[[package]]
+name = "simple-error"
+version = "0.1.11"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
 [[package]]
 name = "strsim"
 version = "0.6.0"
@@ -228,6 +233,7 @@ dependencies = [
  "nom 3.2.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "nom_locate 0.1.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "serde_json 1.0.21 (registry+https://github.com/rust-lang/crates.io-index)",
+ "simple-error 0.1.11 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
@@ -295,6 +301,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 "checksum rustc-demangle 0.1.8 (registry+https://github.com/rust-lang/crates.io-index)" = "76d7ba1feafada44f2d38eed812bd2489a03c0f5abb975799251518b68848649"
 "checksum serde 1.0.66 (registry+https://github.com/rust-lang/crates.io-index)" = "e9a2d9a9ac5120e0f768801ca2b58ad6eec929dc9d1d616c162f208869c2ce95"
 "checksum serde_json 1.0.21 (registry+https://github.com/rust-lang/crates.io-index)" = "eb40600c756f02d7ea34943626cefa85732fdae5f95b90b31f9797b3c526d1e6"
+"checksum simple-error 0.1.11 (registry+https://github.com/rust-lang/crates.io-index)" = "7779d1977a9e1e50bebb430a57114acc64bc4c40d6d8efb3e57893531d5fd895"
 "checksum strsim 0.6.0 (registry+https://github.com/rust-lang/crates.io-index)" = "b4d15c810519a91cf877e7e36e63fe068815c678181439f2f29e2562147c3694"
 "checksum term_size 0.3.1 (registry+https://github.com/rust-lang/crates.io-index)" = "9e5b9a66db815dcfd2da92db471106457082577c3c278d4138ab3e3b4e189327"
 "checksum termion 1.5.1 (registry+https://github.com/rust-lang/crates.io-index)" = "689a3bdfaab439fd92bc87df5c4c78417d3cbe537487274e9b0b2dce76e92096"
diff --git a/Cargo.toml b/Cargo.toml
index 7276581..7e74e29 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -13,9 +13,10 @@ license     = "Apache-2.0"
 version        = "^3.2"
 
 [dependencies]
-nom_locate  = "^0.1.1"
-clap        = "~2.26.0"
-serde_json  = "~1.0.9"
+nom_locate   = "^0.1.1"
+clap         = "~2.26.0"
+serde_json   = "~1.0.9"
+simple-error = "0.1"
 
 [dev-dependencies]
 bencher = "~0.1.5"
diff --git a/src/build/mod.rs b/src/build/mod.rs
index b20a69d..49b75c1 100644
--- a/src/build/mod.rs
+++ b/src/build/mod.rs
@@ -488,16 +488,25 @@ impl Builder {
         eprintln!("processing import for {}", normalized.to_string_lossy());
         // Only parse the file once on import.
         let mut shared_assets = self.assets.borrow_mut();
-        if try!(shared_assets.get(&normalized)).is_some() {
-            return Ok(try!(shared_assets.get(&normalized)).unwrap().clone());
+        let result = match try!(shared_assets.get(&normalized)) {
+            Some(v) => v.clone(),
+            None => {
+                let mut b = Self::new(normalized.clone(), self.assets.clone());
+                let filepath = normalized.to_str().unwrap().clone();
+                try!(b.build_file(filepath));
+                let fields: Vec<(Positioned<String>, Rc<Val>)> = b.build_output.drain().collect();
+                Rc::new(Val::Tuple(fields))
+            }
+        };
+        let key = sym.into();
+        if self.build_output.contains_key(&key) {
+            return Err(Box::new(error::Error::new(
+                format!("Binding for import name {} already exists", sym.fragment),
+                error::ErrorType::DuplicateBinding,
+                def.path.pos.clone(),
+            )));
         }
-        let mut b = Self::new(normalized.clone(), self.assets.clone());
-        let filepath = normalized.to_str().unwrap().clone();
-        try!(b.build_file(filepath));
-        let fields: Vec<(Positioned<String>, Rc<Val>)> = b.build_output.drain().collect();
-        let result = Rc::new(Val::Tuple(fields));
-        //eprintln!("storing sym {:?} results {:?} ", sym, result)
-        self.build_output.insert(sym.into(), result.clone());
+        self.build_output.insert(key, result.clone());
         try!(shared_assets.stash(normalized.clone(), result.clone()));
         return Ok(result);
     }
diff --git a/src/lib.rs b/src/lib.rs
index 64d5dea..006b0f3 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -423,6 +423,7 @@ extern crate nom;
 #[macro_use]
 extern crate nom_locate;
 extern crate serde_json;
+extern crate simple_error;
 
 #[macro_use]
 pub mod ast;
diff --git a/src/main.rs b/src/main.rs
index dd9b457..732e34e 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -59,16 +59,19 @@ fn run_converter(c: ConverterRunner, v: Rc<Val>, f: Option<&str>) -> traits::Res
 
 fn main() {
     let app = do_flags();
+    let cache = Rc::new(RefCell::new(MemoryCache::new()));
     if let Some(matches) = app.subcommand_matches("build") {
         let file = matches.value_of("INPUT").unwrap();
         let out = matches.value_of("out");
         let sym = matches.value_of("sym");
         let target = matches.value_of("target").unwrap();
         let root = PathBuf::from(file);
-        let cache = Rc::new(RefCell::new(MemoryCache::new()));
         let mut builder = build::Builder::new(root.parent().unwrap(), cache);
         match ConverterRunner::new(target) {
             Ok(converter) => {
+                // FIXME(jwall): What should happen if they requested we build a _test.ucg file.
+                // 1. We could automatically go into validate mode.
+                // 2. We could warn that this is a test file.
                 let result = builder.build_file(file);
                 if !result.is_ok() {
                     eprintln!("{:?}", result.err().unwrap());
@@ -97,7 +100,6 @@ fn main() {
         }
     } else if let Some(matches) = app.subcommand_matches("validate") {
         let file = matches.value_of("INPUT").unwrap();
-        let cache = Rc::new(RefCell::new(MemoryCache::new()));
         let mut builder = build::Builder::new(std::env::current_dir().unwrap(), cache);
         builder.enable_validate_mode();
         builder.build_file(file).unwrap();