diff --git a/sim/mcuboot-sys/build.rs b/sim/mcuboot-sys/build.rs
index ad33483..7db37f1 100644
--- a/sim/mcuboot-sys/build.rs
+++ b/sim/mcuboot-sys/build.rs
@@ -27,7 +27,6 @@
         conf.define("MCUBOOT_SIGN_RSA", None);
         conf.define("MCUBOOT_USE_MBED_TLS", None);
 
-        conf.define("MCUBOOT_USE_MBED_TLS", None);
         conf.define("MBEDTLS_CONFIG_FILE", Some("<config-boot.h>"));
         conf.include("mbedtls/include");
         conf.file("mbedtls/library/sha256.c");