The problem is that obj files contain vertex data in parts:pos,tex,normal and indices in them point to each of these parts individually. In OpenGL you need to combine those parts into one data object - vertex. And have your indices point to vertices, not their parts.

My code (scala, but should be ok)

val vertexListB = new RList[vec3]()
val texListB = new RList[vec2]()
val normalListB = new RList[vec3]()
val indexListB = new RList[Int]()

val vertices = new RList[VertexFull]()

for(i <- 0 until vertexIndicesTriangleList.size()){
  val vI = vertexIndicesTriangleList(i) //vertexIndicesTriangleList - all scanned vertex positions from obj file
  val tI = texCoordsIndicesTriangleList(i) //texCoordsIndicesTriangleList - all scanned vertex texture coords from obj file
  val nI = normalIndicesTriangleList(i)//normalIndicesTriangleList - all scanned vertex normals from obj file
  //we are looping through all vertex positions

  val v = vertexList(vI) //some pos
  val t = textureList(tI)//some texCoord that corresponds to that pos
  val n = normalList(nI)//some normal that corresponds to that pos

  val hv = vertexListB.getIndex(v,false)
  val ht = texListB.getIndex(t, false)
  val hn = normalListB.getIndex(n, false)

  if(ht != -1 && hv == ht && hn == ht){ //check for duplicate vertex data, its duplicated if and only if all of 3 components:pos, tex and normal are equal
    indexListB << hv //if ht != -1 then our list doesnt contain that vertex yet
  }
  else{
    vertexListB << v //if doesnt contain/not duplicated then put data into the lists
    texListB << t
    normalListB << n
    indexListB << vertexListB.size() - 1//index points to the last put vertex, vertexListB, texListB, normalListB are equal sized
  }

}

for(i <- 0 until vertexListB.size()){
  vertices << new VertexFull(vertexListB(i), texListB(i), normalListB(i))
}

The problem is that obj files contain vertex data in parts:pos,tex,normal and indices in them point to each of these parts individually. In OpenGL you need to combine those parts into one data object - vertex. And have your indices point to vertices, not their parts.

My code (scala, but should be ok)

val vertexListB = new RList[vec3]()//output positions
val texListB = new RList[vec2]()//output texCoords
val normalListB = new RList[vec3]()//output normals
val indexListB = new RList[Int]()//output indices

val vertices = new RList[VertexFull]()//output vertex structure list, consists of pos, tex, normal

for(i <- 0 until vertexIndicesTriangleList.size()){
  val vI = vertexIndicesTriangleList(i) //vertexIndicesTriangleList - all scanned vertex positions from obj file
  val tI = texCoordsIndicesTriangleList(i) //texCoordsIndicesTriangleList - all scanned vertex texture coords from obj file
  val nI = normalIndicesTriangleList(i)//normalIndicesTriangleList - all scanned vertex normals from obj file
  //we are looping through all vertex positions

  val v = vertexList(vI) //some pos
  val t = textureList(tI)//some texCoord that corresponds to that pos
  val n = normalList(nI)//some normal that corresponds to that pos

  val hv = vertexListB.getIndex(v,false)
  val ht = texListB.getIndex(t, false)
  val hn = normalListB.getIndex(n, false)

  if(ht != -1 && hv == ht && hn == ht){ //check for duplicate vertex data, its duplicated if and only if all of 3 components:pos, tex and normal are equal
    indexListB << hv //if ht != -1 then our list does contain that vertex already
  }
  else{
    vertexListB << v //if doesnt contain/not duplicated then put data into the lists
    texListB << t
    normalListB << n
    indexListB << vertexListB.size() - 1//index points to the last put vertex, vertexListB, texListB, normalListB are equal sized
  }

}

for(i <- 0 until vertexListB.size()){
  vertices << new VertexFull(vertexListB(i), texListB(i), normalListB(i))
}