What is important to focus on in this scene is the way in which Shylock has just suffered the loss of everything that is important to him in life: his daughter, Jessica, has eloped with the Christian Lorenzo, and has also stolen all of his wealth when she left. He is a man desolate and empty. Thus it is that the only thing he has to cling on to in life is the "ancient grudge" that he has against Christians and in particular, Antonio. The "merry bond" that he made with Antonio is now his only focus and the way that he obsesses about it and single-mindedly looks toward the time when he can gain his revenge for all of the tragedies he has suffered is underlined through the repetition of the phrase you have indicated. Note what he says to Salerio and Solanio:
A bankrupt, a prodigal, who dare scarce show his head on the Rialto; a beggar, that was used to come so smug upon the mart! let him look to his bond. He was wont to call me usurer. Let him look to his bond. He was wont to lend money for a Christian courtesy. Let him look to his bond.
Note the way that before each repetition of the phrase Shylock cites yet another grievance he has against Antonio. The repeptition of the phrase "Let him look to his bond" seems to underscore the inevitability of the judgement that Shylock anticipates exacting against his enemy and his keen anticipation in doing so.
Marchette Chute(STORIES FROM SHAKESPEARE) wrote: "THE MERCHANT OF VENICE is a romantic comedy, but of a most unusual kind. For the theme is money,...." The editors of the recent RSC edition noted "the language of money that suffuses the whole play." In the first scene, Antonio denies that thought of his merchandise is the cause of his sadness. So, other possible causes are noted. In my dictionary I find meaning number three for the word "bond" reads: "A uniting force or tie; link: bonds of friendship." Therefore, the repitiition might recall Act 1, scene 3 where we find Shylock's "I say / To buy his favor I extend this friendship. / if he will take it, so; if not, adieu. / And for my love, I pray you, wrong me not" and Antonio's reply: "Yes, Shylock, I will seal unto this bond"(1.3.166-170 or so).
You can look at it in another way too... Shylock is also a human being who is capable of showing mercy. In order to prevent himself from showing mercy, he stresses on the bond.