"Grace, grace" - In the Hebrew, this is literally "favor, favor." Perhaps more literally, it should be rendered "beauty, beauty." In the Psalms, there is a concept called "the beauty of holiness" (see Psalm 29:2; 96:9; 110:3), which is connected to this "grace, grace." However, this instance is a reiteration of what is said in verse 6, that it is only by God's gifts and favor that the work will be accomplished.
The Hebrew language repeats itself a great deal—putting ideas in a slightly different way, in parallel, so that we can understand just how things are to be understood. What this means is that it is by God's grace—by His Spirit, by His favor, by His gifts, that the Temple will be completed and ready for God to inhabit. Zerubbabel should not fear anything. It will be done through God's help.
However, we have to link this with the lamps around the central pillar, which is the passage's main theme. The prophecy goes beyond Zerubbabel to the New Testament church—specifically, down to the end-time church. The passage progresses toward the Two Witnesses—the two anointed ones—at the end of this chapter. And much like Revelation 10 and 11, it starts out with an early work and ends with their worldwide work. The early work has to do with completing the Temple, and the later work has to do with a witness for God before the whole world.
Richard T. Ritenbaugh
The Two Witnesses (Part 4)