Do not detect clone entry as duplicated content.

If two entries point to the same tuple (cluter_id, blob_id), we must
not report them as duplicated content.

src/zimcheck/checks.cpp

@@ -14,6 +14,7 @@
 #include <mutex>
 #include <thread>
 #include <queue>
+#include <optional>
 #include <zim/archive.h>
 #include <zim/item.h>
 
@@ -113,6 +114,11 @@ SortedMsgParams sortedMsgParams(const MsgParams& msgParams)
   return SortedMsgParams(msgParams.begin(), msgParams.end());
 }
 
+bool areAliases(const zim::Item& i1, const zim::Item& i2)
+{
+    return i1.getClusterIndex() == i2.getClusterIndex() && i1.getBlobIndex() == i2.getBlobIndex();
+}
+
 } // unnamed namespace
 
 namespace JSON
@@ -487,15 +493,22 @@ void ArticleChecker::detect_redundant_articles()
         progress.report();
         auto l = it.second;
         while ( !l.empty() ) {
-            const auto e1 = archive.getEntryByPath(l.front());
+            // The way we have constructed `l`, e1 MUST BE an item
+            const auto e1 = archive.getEntryByPath(l.front()).getItem();
             l.pop_front();
             if ( !l.empty() ) {
-                // The way we have constructed `l`, e1 MUST BE an item
-                const std::string s1 = e1.getItem().getData();
+                std::optional<std::string> s1;
                 decltype(l) articlesDifferentFromE1;
                 for(auto other : l) {
-                    auto e2 = archive.getEntryByPath(other);
-                    std::string s2 = e2.getItem().getData();
+                    // The way we have constructed `l`, e2 MUST BE an item
+                    const auto e2 = archive.getEntryByPath(other).getItem();
+                    if (areAliases(e1, e2)) {
+                        continue;
+                    }
+                    if (!s1) {
+                        s1 = e1.getData();
+                    }
+                    std::string s2 = e2.getData();
                     if (s1 != s2 ) {
                         articlesDifferentFromE1.push_back(other);
                         continue;