diff --git a/mailpoet/lib/Automation/Engine/Validation/WorkflowRules/NoJoinRule.php b/mailpoet/lib/Automation/Engine/Validation/WorkflowRules/NoJoinRule.php
new file mode 100644
index 0000000000..ef552a201b
--- /dev/null
+++ b/mailpoet/lib/Automation/Engine/Validation/WorkflowRules/NoJoinRule.php
@@ -0,0 +1,32 @@
+<?php declare(strict_types = 1);
+
+namespace MailPoet\Automation\Engine\Validation\WorkflowRules;
+
+use MailPoet\Automation\Engine\Data\Step;
+use MailPoet\Automation\Engine\Data\Workflow;
+use MailPoet\Automation\Engine\Exceptions;
+use MailPoet\Automation\Engine\Validation\WorkflowGraph\WorkflowNode;
+use MailPoet\Automation\Engine\Validation\WorkflowGraph\WorkflowNodeVisitor;
+
+class NoJoinRule implements WorkflowNodeVisitor {
+  /** @var array<string, Step> */
+  private $visitedSteps = [];
+
+  public function initialize(Workflow $workflow): void {
+    $this->visitedSteps = [];
+  }
+
+  public function visitNode(Workflow $workflow, WorkflowNode $node): void {
+    $step = $node->getStep();
+    $this->visitedSteps[$step->getId()] = $step;
+    foreach ($step->getNextSteps() as $nextStep) {
+      $nextStepId = $nextStep->getId();
+      if (isset($this->visitedSteps[$nextStepId])) {
+        throw Exceptions::workflowStructureNotValid(__('Path join found in workflow graph', 'mailpoet'));
+      }
+    }
+  }
+
+  public function complete(Workflow $workflow): void {
+  }
+}
diff --git a/mailpoet/tests/unit/Automation/Engine/Validation/WorkflowRules/NoJoinRuleTest.php b/mailpoet/tests/unit/Automation/Engine/Validation/WorkflowRules/NoJoinRuleTest.php
new file mode 100644
index 0000000000..c5e9658861
--- /dev/null
+++ b/mailpoet/tests/unit/Automation/Engine/Validation/WorkflowRules/NoJoinRuleTest.php
@@ -0,0 +1,73 @@
+<?php declare(strict_types = 1);
+
+namespace MailPoet\Automation\Engine\Validation\WorkflowRules;
+
+require_once __DIR__ . '/WorkflowRuleTest.php';
+
+use MailPoet\Automation\Engine\Exceptions\UnexpectedValueException;
+use MailPoet\Automation\Engine\Validation\WorkflowGraph\WorkflowWalker;
+
+class NoJoinRuleTest extends WorkflowRuleTest {
+  public function testItDetectsJoinedPath(): void {
+    $workflow = $this->createWorkflow([
+      'root' => ['a1', 'a2'],
+      'a1' => ['b'],
+      'a2' => ['b'],
+      'b' => [],
+    ]);
+
+    $this->expectException(UnexpectedValueException::class);
+    $this->expectExceptionMessage('Invalid workflow structure: Path join found in workflow graph');
+    (new WorkflowWalker())->walk($workflow, [new NoJoinRule()]);
+  }
+
+  public function testItDetectsLongJoinedPath(): void {
+    $workflow = $this->createWorkflow([
+      'root' => ['a1', 'a2'],
+      'a1' => ['b1'],
+      'a2' => ['b2'],
+      'b1' => ['c1'],
+      'b2' => ['c2'],
+      'c1' => ['d'],
+      'c2' => ['d'],
+      'd' => [],
+    ]);
+
+    $this->expectException(UnexpectedValueException::class);
+    $this->expectExceptionMessage('Invalid workflow structure: Path join found in workflow graph');
+    (new WorkflowWalker())->walk($workflow, [new NoJoinRule()]);
+  }
+
+  public function testItDetectsJoinedPathToSelf(): void {
+    $workflow = $this->createWorkflow([
+      'root' => ['root', 'root'],
+    ]);
+
+    $this->expectException(UnexpectedValueException::class);
+    $this->expectExceptionMessage('Invalid workflow structure: Path join found in workflow graph');
+    (new WorkflowWalker())->walk($workflow, [new NoJoinRule()]);
+  }
+
+  public function testItPassesWithSimplePath(): void {
+    $workflow = $this->createWorkflow([
+      'root' => ['a'],
+      'a' => ['b'],
+      'b' => ['c'],
+      'c' => [],
+    ]);
+
+    (new WorkflowWalker())->walk($workflow, [new NoJoinRule()]);
+    // no exception thrown
+  }
+
+  public function testItPassesWithPathSplit(): void {
+    $workflow = $this->createWorkflow([
+      'root' => ['a1', 'a1'],
+      'a1' => [],
+      'a2' => [],
+    ]);
+
+    (new WorkflowWalker())->walk($workflow, [new NoJoinRule()]);
+    // no exception thrown
+  }
+}