Don't allow two equals for the same key

lexer.go

diff --git a/lexer.go b/lexer.go
index 0ad27a2..679bb4d 100644
--- a/lexer.go
+++ b/lexer.go
@@ -192,6 +192,10 @@
 	for {
 		next := l.peek()
 		switch next {
+		case '.':
+			return l.errorf("cannot start float with a dot")
+		case '=':
+			return l.errorf("cannot have multiple equals for the same key")
 		case '[':
 			l.depth += 1
 			return lexLeftBracket
@@ -234,10 +238,6 @@
 			return lexNumber
 		}
 
-		if next == '.' {
-			return l.errorf("cannot start float with a dot")
-		}
-
 		if isSpace(next) {
 			l.ignore()
 		}

lexer_test.go

diff --git a/lexer_test.go b/lexer_test.go
index c2bb2f2..e3b5daf 100644
--- a/lexer_test.go
+++ b/lexer_test.go
@@ -284,6 +284,14 @@
 	})
 }
 
+func TestDoubleEqualKey(t *testing.T) {
+	testFlow(t, "foo= = 2", []token{
+		token{tokenKey, "foo"},
+		token{tokenEqual, "="},
+		token{tokenError, "cannot have multiple equals for the same key"},
+	})
+}
+
 func TestKeyEqualNumber(t *testing.T) {
 	testFlow(t, "foo = 42", []token{
 		token{tokenKey, "foo"},