Using the assimp library for importing 3D data into an OpenGL application. I get some strange problems regarding indexing of the vertices:

If I use the following code for importing vertex indices:

for (unsigned int t = 0; t < mesh->mNumFaces; ++t) 
    {
        const struct aiFace * face = &mesh->mFaces[t];
        if (face->mNumIndices == 3)
        {
        indices->push_back(face->mIndices[0]);
        indices->push_back(face->mIndices[1]);
        indices->push_back(face->mIndices[2]);
        }
    }

I get the following result:

Instead, if I use the following code:

for(int k = 0; k < 2 ; k++)
    {
        for (unsigned int t = 0; t < mesh->mNumFaces; ++t) 
        {
            const struct aiFace * face = &mesh->mFaces[t];
            if (face->mNumIndices == 3)
            {
            indices->push_back(face->mIndices[0]);
            indices->push_back(face->mIndices[1]);
            indices->push_back(face->mIndices[2]);
            }
        }
    }

I get the correct result:

Hence adding the indices twice, renders the correct result?

The OpenGL buffer is populated, like so:

glBufferData(GL_ELEMENT_ARRAY_BUFFER, 
             indices->size() * sizeof(unsigned int), 
             indices->data(), 
             GL_STATIC_DRAW);

And rendered as follows:

glDrawElements(GL_TRIANGLES, 
               vertexCount*3, 
               GL_UNSIGNED_INT, 
               indices->data());