How to detect infinite loop exist in linear bounded automata (LBA)? The following theorem from Michael Sipser's book "Introduction to the Theory of Computation" states:


  $A_{\textrm{LBA}}= \{ \langle M, w \rangle \mid \text{$M$ is an LBA that accepts string $w$} \}$.
  
  THEOREM: $A_{\mathrm{LBA}}$ is decidable.


On the proof part, it states:


  The idea for detecting when $M$ is looping is that as $M$ computes on $w$,
  it goes from configuration to configuration. If $M$ ever repeats a
  configuration, it would go on to repeat this configuration over and
  over again and thus be in a loop.


I do not understand this: 
"If $M$ ever repeats a configuration, it would go on to repeat this configuration over and over again". What if $M$ only repeat one configuration, then halts?
The machine $M$ is deterministic. This means that, if $M$ is in a certain configuration $c$, then there is a single fixed configuration $c'$ (determined by the rules of $M$) which the execution of one step will lead it to. If $M$ ever reaches the configuration $c$ again, then the configuration $c'$ will follow no matter what. Hence, if the computation of $M$ causes it to be assume configurations $c_1, \dots, c_n$ and $c_n = c_1$, then $M$ will repeat the loop $c_1, \ldots, c_n$ indefinitely.