Digital twin (DT) technology has demonstrated advantages in various industries, including manufacturing, service, and healthcare. In particular, the significance of this technology has grown substantially in the manufacturing sector, particularly in light of Industry 4.0 developments. In the past ten years, much research has been conducted on the framework and methods for DT implementation in a manufacturing system. However, the current status of DT implementation remains unclear, particularly about the level of integration, which refers to the extent and direction of data exchange between the physical system and its digital counterpart, and the extent of coverage across manufacturing system components such as machines, tools, workers, and control systems. This study presents a systematic literature review to examine the current status of DT implementation about key manufacturing system components and provides directions for future research. It first explores the purpose and historical background of DTs, as well as the key challenges associated with their implementation. Based on these insights, the study then proposes a four-step planning framework for implementing an intelligent DT in manufacturing systems.