Fix using the same previous data table column multiple times in the same cell (#2083)

Author: Vampire

spock-core/src/main/java/org/spockframework/compiler/WhereBlockRewriter.java

@@ -675,7 +675,7 @@ private void turnIntoSimpleParameterization(List<Expression> column) throws Inva
 
         // otherwise generate the extractors and closure
         List<Statement> statements = new ArrayList<>();
-        List<String> referencedPreviousVariables = previousVariableAccesses.stream().map(VariableExpression::getName).collect(toList());
+        Set<String> referencedPreviousVariables = previousVariableAccesses.stream().map(VariableExpression::getName).collect(toSet());
         generatePreviousColumnExtractorStatements(referencedPreviousVariables, row, statements);
         ReturnStatement providerStatement = new ReturnStatement(providerExpression);
         providerStatement.setSourcePosition(providerExpression);
@@ -702,7 +702,7 @@ private void turnIntoSimpleParameterization(List<Expression> column) throws Inva
     rewriteSimpleParameterization(binExpr, varExpr, true);
   }
 
-  private void generatePreviousColumnExtractorStatements(List<String> referencedPreviousVariables, int row,
+  private void generatePreviousColumnExtractorStatements(Set<String> referencedPreviousVariables, int row,
                                                          List<Statement> statements) {
     for (String referencedPreviousVariable : referencedPreviousVariables) {
       statements.add(new ExpressionStatement(

spock-specs/src/test/groovy/org/spockframework/smoke/ast/DataTablesAstSpec.groovy

@@ -16,6 +16,7 @@ package org.spockframework.smoke.ast
 
 import org.spockframework.EmbeddedSpecification
 import org.spockframework.specs.extension.SpockSnapshotter
+import spock.lang.Issue
 import spock.lang.Snapshot
 import spock.util.Show
 
@@ -83,4 +84,24 @@ class DataTablesAstSpec extends EmbeddedSpecification {
     then:
     snapshotter.assertThat(result.source).matchesSnapshot()
   }
+
+  @Issue('https://github.com/spockframework/spock/issues/2083')
+  def 'using a variable in a cell multiple times compiles'() {
+    given:
+    snapshotter.featureBody()
+
+    when:
+    def result = compiler.transpileFeatureBody '''
+        expect:
+        a + b == result
+
+        where:
+        a | b | result
+        1 | 2 | a + b
+        3 | 4 | a + a // causes the compile error
+    ''', EnumSet.of(Show.METHODS)
+
+    then:
+    snapshotter.assertThat(result.source).matchesSnapshot()
+  }
 }