diff --git a/moto/stepfunctions/exceptions.py b/moto/stepfunctions/exceptions.py
index 704e4ea83..6000bab4e 100644
--- a/moto/stepfunctions/exceptions.py
+++ b/moto/stepfunctions/exceptions.py
@@ -18,6 +18,11 @@ class AWSError(Exception):
         )
 
 
+class ExecutionAlreadyExists(AWSError):
+    TYPE = "ExecutionAlreadyExists"
+    STATUS = 400
+
+
 class ExecutionDoesNotExist(AWSError):
     TYPE = "ExecutionDoesNotExist"
     STATUS = 400
diff --git a/moto/stepfunctions/models.py b/moto/stepfunctions/models.py
index e36598f23..58b6bb434 100644
--- a/moto/stepfunctions/models.py
+++ b/moto/stepfunctions/models.py
@@ -8,6 +8,7 @@ from moto.core.utils import iso_8601_datetime_without_milliseconds
 from moto.sts.models import ACCOUNT_ID
 from uuid import uuid4
 from .exceptions import (
+    ExecutionAlreadyExists,
     ExecutionDoesNotExist,
     InvalidArn,
     InvalidName,
@@ -205,6 +206,7 @@ class StepFunctionBackend(BaseBackend):
 
     def start_execution(self, state_machine_arn, name=None):
         state_machine_name = self.describe_state_machine(state_machine_arn).name
+        self._ensure_execution_name_doesnt_exist(name)
         execution = Execution(
             region_name=self.region_name,
             account_id=self._get_account_id(),
@@ -278,6 +280,13 @@ class StepFunctionBackend(BaseBackend):
         if not arn or not match:
             raise InvalidArn(invalid_msg)
 
+    def _ensure_execution_name_doesnt_exist(self, name):
+        for execution in self.executions:
+            if execution.name == name:
+                raise ExecutionAlreadyExists(
+                    "Execution Already Exists: '" + execution.execution_arn + "'"
+                )
+
     def _get_account_id(self):
         return ACCOUNT_ID
 
diff --git a/requirements-dev.txt b/requirements-dev.txt
index 313f2dfb6..e40a568a5 100644
--- a/requirements-dev.txt
+++ b/requirements-dev.txt
@@ -1,6 +1,6 @@
 -r requirements.txt
 nose
-black; python_version >= '3.6'
+black==19.10b0; python_version >= '3.6'
 regex==2019.11.1; python_version >= '3.6'   # Needed for black
 sure==1.4.11
 coverage==4.5.4
diff --git a/setup.py b/setup.py
index 707a56212..ffaa8b273 100755
--- a/setup.py
+++ b/setup.py
@@ -40,6 +40,7 @@ install_requires = [
     "werkzeug",
     "PyYAML>=5.1",
     "pytz",
+    "ecdsa<0.15",
     "python-dateutil<3.0.0,>=2.1",
     "python-jose[cryptography]>=3.1.0,<4.0.0",
     "docker>=2.5.1",
diff --git a/tests/test_stepfunctions/test_stepfunctions.py b/tests/test_stepfunctions/test_stepfunctions.py
index 4324964d8..043fd9bfb 100644
--- a/tests/test_stepfunctions/test_stepfunctions.py
+++ b/tests/test_stepfunctions/test_stepfunctions.py
@@ -404,6 +404,27 @@ def test_state_machine_start_execution_with_custom_name():
     execution["startDate"].should.be.a(datetime)
 
 
+@mock_stepfunctions
+@mock_sts
+def test_state_machine_start_execution_fails_on_duplicate_execution_name():
+    client = boto3.client("stepfunctions", region_name=region)
+    #
+    sm = client.create_state_machine(
+        name="name", definition=str(simple_definition), roleArn=_get_default_role()
+    )
+    execution_one = client.start_execution(
+        stateMachineArn=sm["stateMachineArn"], name="execution_name"
+    )
+    #
+    with assert_raises(ClientError) as exc:
+        _ = client.start_execution(
+            stateMachineArn=sm["stateMachineArn"], name="execution_name"
+        )
+    exc.exception.response["Error"]["Message"].should.equal(
+        "Execution Already Exists: '" + execution_one["executionArn"] + "'"
+    )
+
+
 @mock_stepfunctions
 @mock_sts
 def test_state_machine_list_executions():